I'm using 0.9.9 snapshot since it's the one required to use for Harvest Scythes.

P.S. Tried running 0.9.8 and the crash doesn't happen.

---- Minecraft Crash Report ----
// Don't be sad, have a hug! <3

Time: 1/5/21 12:02 AM
Description: Exception in server tick loop

org.spongepowered.asm.mixin.transformer.throwables.MixinTransformerError: An unexpected critical error was encountered
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:363)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClass(MixinTransformer.java:208)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClassBytes(MixinTransformer.java:178)
at org.spongepowered.asm.mixin.transformer.FabricMixinTransformerProxy.transformClassBytes(FabricMixinTransformerProxy.java:23)
at net.fabricmc.loader.launch.knot.KnotClassDelegate.getPostMixinClassByteArray(KnotClassDelegate.java:157)
at net.fabricmc.loader.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:150)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at net.minecraft.class_2281.method_9534(class_2281.java:207)
at net.minecraft.class_4970$class_4971.method_26174(class_4970.java:894)
at net.minecraft.class_3225.method_14262(class_3225.java:328)
at net.minecraft.class_3244.method_12046(class_3244.java:1040)
at net.minecraft.class_2885.method_12547(class_2885.java:36)
at net.minecraft.class_2885.method_11054(class_2885.java:10)
at net.minecraft.class_2600.method_11072(class_2600.java:21)
at net.minecraft.class_2600$$Lambda$7457/35485522.run(Unknown Source)
at net.minecraft.class_3738.run(class_3738.java:18)
at net.minecraft.class_1255.method_18859(class_1255.java:144)
at net.minecraft.class_4093.method_18859(class_4093.java:23)
at net.minecraft.server.MinecraftServer.method_24306(MinecraftServer.java:761)
at net.minecraft.server.MinecraftServer.method_18859(MinecraftServer.java:155)
at net.minecraft.class_1255.method_16075(class_1255.java:118)
at net.minecraft.server.MinecraftServer.method_20415(MinecraftServer.java:743)
at net.minecraft.server.MinecraftServer.method_16075(MinecraftServer.java:737)
at net.minecraft.class_1255.method_18857(class_1255.java:127)
at net.minecraft.server.MinecraftServer.method_16208(MinecraftServer.java:722)
at net.minecraft.server.MinecraftServer.method_29741(MinecraftServer.java:674)
at net.minecraft.server.MinecraftServer.method_29739(MinecraftServer.java:257)
at net.minecraft.server.MinecraftServer$$Lambda$6830/1627256788.run(Unknown Source)
at java.lang.Thread.run(Thread.java:745)
Caused by: org.spongepowered.asm.mixin.throwables.MixinApplyError: Mixin [piglib.mixins.json:PiglinBrainMixin] from phase [DEFAULT] in config [piglib.mixins.json] FAILED during APPLY
at org.spongepowered.asm.mixin.transformer.MixinProcessor.handleMixinError(MixinProcessor.java:642)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.handleMixinApplyError(MixinProcessor.java:594)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:356)
... 28 more
Caused by: org.spongepowered.asm.mixin.injection.throwables.InvalidInjectionException: Critical injection failure: @Inject annotation on acceptsForBarterInject could not find any targets matching 'Lnet/minecraft/class_4838;method_24746(Lnet/minecraft/class_1799;)Z' in net.minecraft.class_4838. Using refmap piglib-refmap.json [PREINJECT Applicator Phase -> piglib.mixins.json:PiglinBrainMixin -> Prepare Injections -> -> handler$zfh000$acceptsForBarterInject(Lnet/minecraft/class_1799;Lorg/spongepowered/asm/mixin/injection/callback/CallbackInfoReturnable;)V -> Parse]
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo.findMethods(InjectionInfo.java:572)
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo.readAnnotation(InjectionInfo.java:288)
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo.(InjectionInfo.java:275)
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo.(InjectionInfo.java:267)
at org.spongepowered.asm.mixin.injection.struct.CallbackInjectionInfo.(CallbackInjectionInfo.java:46)
at sun.reflect.GeneratedConstructorAccessor13.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:422)
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo$InjectorEntry.create(InjectionInfo.java:140)
at org.spongepowered.asm.mixin.injection.struct.InjectionInfo.parse(InjectionInfo.java:624)
at org.spongepowered.asm.mixin.transformer.MixinTargetContext.prepareInjections(MixinTargetContext.java:1247)
at org.spongepowered.asm.mixin.transformer.MixinApplicatorStandard.prepareInjections(MixinApplicatorStandard.java:1033)
at org.spongepowered.asm.mixin.transformer.MixinApplicatorStandard.applyMixin(MixinApplicatorStandard.java:388)
at org.spongepowered.asm.mixin.transformer.MixinApplicatorStandard.apply(MixinApplicatorStandard.java:320)
at org.spongepowered.asm.mixin.transformer.TargetClassContext.applyMixins(TargetClassContext.java:345)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:569)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:351)
... 28 more

A detailed walkthrough of the error, its code path and all known details is as follows:

-- System Details --
Details:
Minecraft Version: 1.16.4
Minecraft Version ID: 1.16.4
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_51, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 1947000368 bytes (1856 MB) / 6442450944 bytes (6144 MB) up to 6442450944 bytes (6144 MB)
CPUs: 6
JVM Flags: 9 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xss1M -Xmx6G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
Fabric Mods:
adorn: Adorn 1.12.2+1.16.4
autoconfig1u: Auto Config v1 Updated 3.3.1
betterend: Better End 0.8.5-beta
betternether: Better Nether 5.0.4
biomemakeover: Biome Makeover 1.16.4-1.1.6
blue_endless_jankson: jankson 1.1.2
byg: BYG 1.1.3
campanion: Campanion 1.3.1+build.51
canvas: Canvas Renderer 1.0.1267
charm: Charm 2.2.2
charmonium: Charmonium 2.2.2
cinderscapes: Cinderscapes 1.3.0+build.51
cloth-basic-math: Cloth Basic Math 0.5.1
cloth-client-events-v0: Cloth Client Events v0 1.4.5
cloth-config2: Cloth Config v4 4.8.3
com_electronwill_night-config_core: core 3.6.2
com_electronwill_night-config_toml: toml 3.6.2
com_moandjiezana_toml_toml4j: toml4j 0.7.2
dawn: Dawn API 1.6.1
disable_custom_worlds_advice: Disable Custom Worlds Advice 1.2
fabric: Fabric API 0.29.2+1.16
fabric-api-base: Fabric API Base 0.2.0+ab87788d3a
fabric-biome-api-v1: Fabric Biome API (v1) 3.1.0+2e23b97c3a
fabric-blockrenderlayer-v1: Fabric BlockRenderLayer Registration (v1) 1.1.4+6a2618f53a
fabric-command-api-v1: Fabric Command API (v1) 1.0.9+6a2618f53a
fabric-commands-v0: Fabric Commands (v0) 0.2.1+cbe9176f3a
fabric-containers-v0: Fabric Containers (v0) 0.1.9+a03e98793a
fabric-content-registries-v0: Fabric Content Registries (v0) 0.2.0+e77439c73a
fabric-crash-report-info-v1: Fabric Crash Report Info (v1) 0.1.2+b7f9825d3a
fabric-dimensions-v1: fabric-dimensions-v1 2.0.1+9a6c75813a
fabric-entity-events-v1: Fabric Entity Events (v1) 1.0.2+8004b36d3a
fabric-events-interaction-v0: Fabric Events Interaction (v0) 0.4.1+6a2618f53a
fabric-events-lifecycle-v0: Fabric Events Lifecycle (v0) 0.2.0+6a2618f53a
fabric-game-rule-api-v1: Fabric Game Rule API (v1) 1.0.5+cbe9176f3a
fabric-item-api-v1: Fabric Item API (v1) 1.2.0+6a2618f53a
fabric-item-groups-v0: Fabric Item Groups (v0) 0.2.2+cbe9176f3a
fabric-key-binding-api-v1: Fabric Key Binding API (v1) 1.0.1+730711c63a
fabric-keybindings-v0: Fabric Key Bindings (v0) 0.2.0+6a2618f53a
fabric-language-kotlin: Fabric Language Kotlin 1.4.21+build.1
fabric-lifecycle-events-v1: Fabric Lifecycle Events (v1) 1.2.0+ffb68a873a
fabric-loot-tables-v1: Fabric Loot Tables (v1) 1.0.1+6a2618f53a
fabric-mining-levels-v0: Fabric Mining Levels (v0) 0.1.2+6a2618f53a
fabric-models-v0: Fabric Models (v0) 0.2.0+cbe9176f3a
fabric-networking-api-v1: Fabric Networking API (v1) 1.0.0+4358fbc63a
fabric-networking-blockentity-v0: Fabric Networking Block Entity (v0) 0.2.7+a03e98793a
fabric-networking-v0: Fabric Networking (v0) 0.3.1+2a4333d33a
fabric-object-builder-api-v1: Fabric Object Builder API (v1) 1.9.2+6a2618f53a
fabric-object-builders-v0: Fabric Object Builders (v0) 0.7.1+6a2618f53a
fabric-particles-v1: Fabric Particles (v1) 0.2.3+cbe9176f3a
fabric-registry-sync-v0: Fabric Registry Sync (v0) 0.7.3+be155ae23a
fabric-renderer-api-v1: Fabric Renderer API (v1) 0.4.0+cbe9176f3a
fabric-renderer-indigo: Fabric Renderer - Indigo 0.4.3+6a2618f53a
fabric-renderer-registries-v1: Fabric Renderer Registries (v1) 2.2.0+6a2618f53a
fabric-rendering-data-attachment-v1: Fabric Rendering Data Attachment (v1) 0.1.4+6a2618f53a
fabric-rendering-fluids-v1: Fabric Rendering Fluids (v1) 0.1.12+6a2618f53a
fabric-rendering-v0: Fabric Rendering (v0) 1.1.1+6a2618f53a
fabric-rendering-v1: Fabric Rendering (v1) 1.5.0+c26373133a
fabric-resource-loader-v0: Fabric Resource Loader (v0) 0.4.0+552549d53a
fabric-screen-handler-api-v1: Fabric Screen Handler API (v1) 1.1.0+6a2618f53a
fabric-structure-api-v1: Fabric Structure API (v1) 1.1.3+cbe9176f3a
fabric-tag-extensions-v0: Fabric Tag Extensions (v0) 1.1.0+e77439c73a
fabric-textures-v0: Fabric Textures (v0) 1.0.5+6a2618f53a
fabric-tool-attribute-api-v1: Fabric Tool Attribute API (v1) 1.2.5+6a2618f53a
fabricloader: Fabric Loader 0.10.8
fermion: Fermion 2.9.218
fermion-orientation: Fermion Orientation 1.2.218
fermion-varia: Fermion Varia 2.6.218
fiber: fiber 0.23.0-2
frex: FREX 4.5.201
frex-events: FREX Events 1.3.13
jankson: Jankson 3.0.1+j1.2.0
json-model-extensions: JSON Model Extensions 1.19.172
libcd: LibCapableData 3.0.3+1.16.3
libgui: LibGui 3.1.0+1.16.3
libstructure: LibStructure 1.5
lithium: Lithium 0.6.0
lithium-api: Lithium API 0.6.0
magna: Magna 1.4.2-1.16.2
minecraft: Minecraft 1.16.4
modmenu: Mod Menu 1.14.13+build.19
mythicmetals: Mythic Metals 0.9.9-snapshot
nookbuild: NookBuild 1.2.1
org_jetbrains_annotations: annotations 20.0.0
org_jetbrains_kotlin_kotlin-reflect: kotlin-reflect 1.4.21
org_jetbrains_kotlin_kotlin-stdlib: kotlin-stdlib 1.4.21
org_jetbrains_kotlin_kotlin-stdlib-jdk7: kotlin-stdlib-jdk7 1.4.21
org_jetbrains_kotlin_kotlin-stdlib-jdk8: kotlin-stdlib-jdk8 1.4.21
org_jetbrains_kotlinx_kotlinx-coroutines-core-jvm: kotlinx-coroutines-core-jvm 1.4.2
org_jetbrains_kotlinx_kotlinx-coroutines-jdk8: kotlinx-coroutines-jdk8 1.4.2
patchouli: Patchouli 1.16.4-48-FABRIC
phosphor: Phosphor 0.7.0+build.10
piglib: Piglib 1.0.1
reach-entity-attributes: Reach Entity Attribute 1.0.1
reborncore: Reborn Core 4.6.3+build.110
respackopts: Resource Pack Options 1.3.1
roughlyenoughitems: Roughly Enough Items 5.8.10
roughlyenoughitems-api: REI (API) 5.8.10
roughlyenoughitems-default-plugin: REI (Default Plugin) 5.8.10
roughlyenoughitems-runtime: REI (Runtime) 5.8.10
special-circumstances: Special Circumstances 1.8.218
static-content: Static Content 1.0.1-1.16.2
staticdata: Static Data 1.1.2
team_reborn_energy: Energy 0.1.1
techreborn: Tech Reborn 3.7.3+build.192
terraform-biome-builder-api-v1: Terraform Biome Builder API (v1) 1.0.0+build.1
terraform-config-api-v1: Terraform Config API (v1) 1.0.0+build.1
terraform-dirt-api-v1: Terraform Dirt API (v1) 1.0.0+build.1
terraform-overworld-biome-extensions-api-v1: Terraform Overworld Biome Extensions API (v1) 1.0.0+build.1
terraform-shapes-api-v1: Terraform Shapes API (v1) 1.0.0+build.4
terraform-surfaces-api-v1: Terraform Surfaces API (v1) 1.0.0+build.1
terraform-tree-api-v1: Terraform Tree API (v1) 1.0.1+build.4
terraform-wood-api-v1: Terraform Wood API (v1) 1.0.0+build.1
terrestria: Terrestria 2.1.5+build.28
tk_valoeghese_zoesteriaconfig: ZoesteriaConfig 1.3.2
traverse: Traverse 3.2.0+build.17
upstream: Upstream 1.0.1
vanilla-hammers: Vanilla Hammers 3.0.1-1.16.3
vanillaexcavators: Vanilla Excavators 1.1.0
wild_explorer: Wild Explorer 1.3.1
zoob: Monster Of The Ocean Depths 2.5.0
Patchouli open book context: n/a
Player Count: 1 / 8; [class_3222['BountyGoblin'/150, l='ServerLevel[New World]', x=63.42, y=70.00, z=2120.59]]
Data Packs: vanilla (incompatible), Fabric Mods
Type: Integrated Server (map_client.txt)
Is Modded: Definitely; Client brand changed to 'fabric'

Update PigLib. The bundled version in 0.9.9 is outdated thanks to someone (cough @LordDeatHunter ) including the wrong version. Might throw up another snapshot soon. Please avoid using this version for now, it is not stable.