The mod in version 1.16.5 requires citadel 1.10.5 (which is for 1.18) or above. The latest version of Citadel for 1.16 is 1.8.1 so it is impossible to load the mod.

Crash log:
`---- Minecraft Crash Report ----
// You're mean.

Time: 4/6/22 2:16 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.server.ServerModLoader.load(ServerModLoader.java:51) ~[forge:?] {re:classloading}
at net.minecraft.server.Main.main(Main.java:95) ~[?:?] {re:classloading}
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_321] {}
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_321] {}
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_321] {}
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_321] {}
at net.minecraftforge.fml.loading.FMLServerLaunchProvider.lambda$launchService$0(FMLServerLaunchProvider.java:51) ~[forge-1.16.5-36.2.33.jar:36.2] {}
at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-8.1.3.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-8.1.3.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-8.1.3.jar:?] {}
at cpw.mods.modlauncher.Launcher.run(Launcher.java:82) [modlauncher-8.1.3.jar:?] {}
at cpw.mods.modlauncher.Launcher.main(Launcher.java:66) [modlauncher-8.1.3.jar:?] {}
at net.minecraftforge.server.ServerMain$Runner.runLauncher(ServerMain.java:63) [forge-1.16.5-36.2.33.jar:?] {}
at net.minecraftforge.server.ServerMain$Runner.access$100(ServerMain.java:60) [forge-1.16.5-36.2.33.jar:?] {}
at net.minecraftforge.server.ServerMain.main(ServerMain.java:57) [forge-1.16.5-36.2.33.jar:?] {}

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

-- Head --
Thread: main
Stacktrace:
at net.minecraftforge.fml.CrashReportExtender.lambda$dumpModLoadingCrashReport$7(CrashReportExtender.java:88) ~[forge:?] {re:classloading}
-- MOD alexsmobs --
Details:
Mod File: alexsmobs-1.16.0.jar
Failure message: Mod alexsmobs requires citadel 1.10.5 or above
Currently, citadel is 1.8.1
Mod Version: 1.16.0
Mod Issue URL: https://github.com/Alex-the-666/AlexsMobs/issues
Exception message: MISSING EXCEPTION MESSAGE
Stacktrace:
at net.minecraftforge.fml.CrashReportExtender.lambda$dumpModLoadingCrashReport$7(CrashReportExtender.java:88) ~[forge:?] {re:classloading}
at java.util.ArrayList.forEach(Unknown Source) ~[?:1.8.0_321] {}
at net.minecraftforge.fml.CrashReportExtender.dumpModLoadingCrashReport(CrashReportExtender.java:86) ~[forge:?] {re:classloading}
at net.minecraftforge.fml.server.ServerModLoader.load(ServerModLoader.java:51) ~[forge:?] {re:classloading}
at net.minecraft.server.Main.main(Main.java:95) ~[?:?] {re:classloading}
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_321] {}
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_321] {}
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_321] {}
at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_321] {}
at net.minecraftforge.fml.loading.FMLServerLaunchProvider.lambda$launchService$0(FMLServerLaunchProvider.java:51) ~[forge-1.16.5-36.2.33.jar:36.2] {}
at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-8.1.3.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-8.1.3.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-8.1.3.jar:?] {}
at cpw.mods.modlauncher.Launcher.run(Launcher.java:82) [modlauncher-8.1.3.jar:?] {}
at cpw.mods.modlauncher.Launcher.main(Launcher.java:66) [modlauncher-8.1.3.jar:?] {}
at net.minecraftforge.server.ServerMain$Runner.runLauncher(ServerMain.java:63) [forge-1.16.5-36.2.33.jar:?] {}
at net.minecraftforge.server.ServerMain$Runner.access$100(ServerMain.java:60) [forge-1.16.5-36.2.33.jar:?] {}
at net.minecraftforge.server.ServerMain.main(ServerMain.java:57) [forge-1.16.5-36.2.33.jar:?] {}

-- System Details --
Details:
Minecraft Version: 1.16.5
Minecraft Version ID: 1.16.5
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_321, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 9197236080 bytes (8771 MB) / 12192317440 bytes (11627 MB) up to 12192317440 bytes (11627 MB)
CPUs: 12
JVM Flags: 2 total; -Xms8G -Xmx12G
ModLauncher: 8.1.3+8.1.3+main-8.1.x.c94d18ec
ModLauncher launch target: fmlserver
ModLauncher naming: srg
ModLauncher services:
/mixin-0.8.4.jar mixin PLUGINSERVICE
/eventbus-4.0.0.jar eventbus PLUGINSERVICE
/forge-1.16.5-36.2.33.jar object_holder_definalize PLUGINSERVICE
/forge-1.16.5-36.2.33.jar runtime_enum_extender PLUGINSERVICE
/accesstransformers-3.0.1.jar accesstransformer PLUGINSERVICE
/forge-1.16.5-36.2.33.jar capability_inject_definalize PLUGINSERVICE
/forge-1.16.5-36.2.33.jar runtimedistcleaner PLUGINSERVICE
/mixin-0.8.4.jar mixin TRANSFORMATIONSERVICE
/forge-1.16.5-36.2.33.jar fml TRANSFORMATIONSERVICE
FML: 36.2
Forge: net.minecraftforge:36.2.33
FML Language Providers:
[email protected]
minecraft@1
[email protected]
Mod List:
forge-1.16.5-36.2.33-server.jar |Minecraft |minecraft |1.16.5 |NONE |Manifest: NOSIGNATURE
forge-1.16.5-36.2.33-universal.jar |Forge |forge |36.2.33 |NONE |Manifest: 22:af:21:d8:19:82:7f:93:94:fe:2b:ac:b7:e4:41:57:68:39:87:b1:a7:5c:c6:44:f9:25:74:21:14:f5:0d:90`