Citadel has class loading errors
java.lang.NoSuchMethodError:

---- Minecraft Crash Report ----
// Uh... Did I do that?

Time: 12/16/21 11:00 AM
Description: Mod loading error has occurred

java.lang.Exception: Mod Loading has failed
at net.minecraftforge.fml.CrashReportExtender.dumpModLoadingCrashReport(CrashReportExtender.java:85) ~[forge:?] {re:classloading}
at net.minecraftforge.fml.client.ClientModLoader.completeModLoading(ClientModLoader.java:188) ~[forge:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft.lambda$null$1(Minecraft.java:508) ~[?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:A,pl:runtimedistcleaner:A}
at net.minecraft.util.Util.func_215077_a(Util.java:320) ~[?:?] {re:classloading}
at net.minecraft.client.Minecraft.lambda$new$2(Minecraft.java:504) ~[?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:A,pl:runtimedistcleaner:A}
at net.minecraft.client.gui.ResourceLoadProgressGui.func_230430_a_(ResourceLoadProgressGui.java:113) ~[?:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.renderer.GameRenderer.func_195458_a(GameRenderer.java:481) ~[?:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft.func_195542_b(Minecraft.java:977) ~[?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:A,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:607) ~[?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:A,pl:runtimedistcleaner:A}
at net.minecraft.client.main.Main.main(Main.java:184) ~[minecraft-1.16.5-client.jar:?] {re:classloading,pl:runtimedistcleaner:A}
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_301] {}
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_301] {}
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_301] {}
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_301] {re:computing_frames}
at net.minecraftforge.fml.loading.FMLClientLaunchProvider.lambda$launchService$0(FMLClientLaunchProvider.java:51) ~[forge-1.16.5-36.2.20-launcher.jar:36.2] {}
at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.Launcher.run(Launcher.java:82) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.Launcher.main(Launcher.java:66) [modlauncher-8.0.9.jar:?] {}
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_301] {}
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_301] {}
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_301] {}
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_301] {re:computing_frames}
at io.github.zekerzhayard.forgewrapper.installer.Main.main(Main.java:53) [ForgeWrapper-mmc1.jar:mmc1] {}
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_301] {}
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_301] {}
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_301] {}
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_301] {re:computing_frames}
at org.multimc.onesix.OneSixLauncher.launchWithMainClass(OneSixLauncher.java:210) [NewLaunch.jar:?] {}
at org.multimc.onesix.OneSixLauncher.launch(OneSixLauncher.java:245) [NewLaunch.jar:?] {}
at org.multimc.EntryPoint.listen(EntryPoint.java:143) [NewLaunch.jar:?] {}
at org.multimc.EntryPoint.main(EntryPoint.java:34) [NewLaunch.jar:?] {}

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

-- Head --
Thread: Render thread
Stacktrace:
at com.github.alexthe666.citadel.Citadel.(Citadel.java:64) ~[citadel:?] {re:classloading}
-- MOD citadel --
Details:
Mod File: citadel-1.8.0-deobf.jar
Failure message: Citadel has class loading errors
java.lang.NoSuchMethodError: net.minecraft.item.Item$Properties.maxStackSize(I)Lnet/minecraft/item/Item$Properties;
Mod Version: 1.8.0
Mod Issue URL: https://github.com/Alex-the-666/Citadel/issues
Exception message: java.lang.NoSuchMethodError: net.minecraft.item.Item$Properties.maxStackSize(I)Lnet/minecraft/item/Item$Properties;
Stacktrace:
at com.github.alexthe666.citadel.Citadel.(Citadel.java:64) ~[citadel:?] {re:classloading}
at java.lang.Class.forName0(Native Method) ~[?:1.8.0_301] {}
at java.lang.Class.forName(Unknown Source) ~[?:1.8.0_301] {}
at net.minecraftforge.fml.javafmlmod.FMLModContainer.(FMLModContainer.java:61) ~[forge:36.2] {re:classloading}
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[?:1.8.0_301] {}
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source) ~[?:1.8.0_301] {}
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source) ~[?:1.8.0_301] {}
at java.lang.reflect.Constructor.newInstance(Unknown Source) ~[?:1.8.0_301] {}
at net.minecraftforge.fml.javafmlmod.FMLJavaModLanguageProvider$FMLModTarget.loadMod(FMLJavaModLanguageProvider.java:76) ~[forge:36.2] {re:classloading}
at net.minecraftforge.fml.ModLoader.buildModContainerFromTOML(ModLoader.java:302) ~[forge:?] {re:classloading}
at net.minecraftforge.fml.ModLoader.lambda$buildMods$29(ModLoader.java:281) ~[forge:?] {re:classloading}
at java.util.stream.ReferencePipeline$3$1.accept(Unknown Source) ~[?:1.8.0_301] {}
at java.util.HashMap$EntrySpliterator.forEachRemaining(Unknown Source) ~[?:1.8.0_301] {}
at java.util.stream.AbstractPipeline.copyInto(Unknown Source) ~[?:1.8.0_301] {}
at java.util.stream.AbstractPipeline.wrapAndCopyInto(Unknown Source) ~[?:1.8.0_301] {}
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(Unknown Source) ~[?:1.8.0_301] {}
at java.util.stream.AbstractPipeline.evaluate(Unknown Source) ~[?:1.8.0_301] {}
at java.util.stream.ReferencePipeline.collect(Unknown Source) ~[?:1.8.0_301] {}
at net.minecraftforge.fml.ModLoader.buildMods(ModLoader.java:283) ~[forge:?] {re:classloading}
at net.minecraftforge.fml.ModLoader.lambda$gatherAndInitializeMods$11(ModLoader.java:183) ~[forge:?] {re:classloading}
at java.util.stream.ReferencePipeline$3$1.accept(Unknown Source) ~[?:1.8.0_301] {}
at java.util.stream.ReferencePipeline$3$1.accept(Unknown Source) ~[?:1.8.0_301] {}
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(Unknown Source) ~[?:1.8.0_301] {}
at java.util.stream.AbstractPipeline.copyInto(Unknown Source) ~[?:1.8.0_301] {}
at java.util.stream.AbstractPipeline.wrapAndCopyInto(Unknown Source) ~[?:1.8.0_301] {}
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(Unknown Source) ~[?:1.8.0_301] {}
at java.util.stream.AbstractPipeline.evaluate(Unknown Source) ~[?:1.8.0_301] {}
at java.util.stream.ReferencePipeline.collect(Unknown Source) ~[?:1.8.0_301] {}
at net.minecraftforge.fml.ModLoader.gatherAndInitializeMods(ModLoader.java:185) ~[forge:?] {re:classloading}
at net.minecraftforge.fml.client.ClientModLoader.lambda$begin$1(ClientModLoader.java:103) ~[forge:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraftforge.fml.client.ClientModLoader.lambda$createRunnableWithCatch$4(ClientModLoader.java:123) ~[forge:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraftforge.fml.client.ClientModLoader.begin(ClientModLoader.java:103) ~[forge:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft.(Minecraft.java:437) ~[?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:A,pl:runtimedistcleaner:A}
at net.minecraft.client.main.Main.main(Main.java:149) ~[minecraft-1.16.5-client.jar:?] {re:classloading,pl:runtimedistcleaner:A}
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_301] {}
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_301] {}
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_301] {}
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_301] {re:computing_frames}
at net.minecraftforge.fml.loading.FMLClientLaunchProvider.lambda$launchService$0(FMLClientLaunchProvider.java:51) ~[forge-1.16.5-36.2.20-launcher.jar:36.2] {}
at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.Launcher.run(Launcher.java:82) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.Launcher.main(Launcher.java:66) [modlauncher-8.0.9.jar:?] {}
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_301] {}
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_301] {}
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_301] {}
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_301] {re:computing_frames}
at io.github.zekerzhayard.forgewrapper.installer.Main.main(Main.java:53) [ForgeWrapper-mmc1.jar:mmc1] {}
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_301] {}
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_301] {}
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_301] {}
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_301] {re:computing_frames}
at org.multimc.onesix.OneSixLauncher.launchWithMainClass(OneSixLauncher.java:210) [NewLaunch.jar:?] {}
at org.multimc.onesix.OneSixLauncher.launch(OneSixLauncher.java:245) [NewLaunch.jar:?] {}
at org.multimc.EntryPoint.listen(EntryPoint.java:143) [NewLaunch.jar:?] {}
at org.multimc.EntryPoint.main(EntryPoint.java:34) [NewLaunch.jar:?] {}

I was having the same problem and found that if I used citadel-1.8.1-1.16.5 rather than citadel-1.8.1-deobf fixed the problem. So, if you see an error like this, it may be that you need to get the version of citadel that matches your forge version #.