Doesn't work on version minecraft purpur 1.21 writes to the console:
[16:01:17] [Server thread/INFO]: [ProtocolLib] Loading server plugin ProtocolLib v5.2.0-SNAPSHOT-679
[16:01:17] [Server thread/WARN]: [ProtocolLib] Version (MC: 1.21.0) has not yet been tested! Proceed with caution.
[16:01:24] [Server thread/ERROR]: [ProtocolLib] Assuming package version: v1_21_R1
[16:01:24] [Server thread/ERROR]: [ProtocolLib] INTERNAL ERROR: Cannot load ProtocolLib.
If this problem hasn't already been reported, please open a ticket
at https://github.com/dmulloy2/ProtocolLib/issues with the following data:
Stack Trace:
java.lang.IllegalArgumentException: Unable to find a field that matches {modifiers=[required: 10000, banned: 1000], type={ type input instanceof interface java.util.Map }}
at ProtocolLib.jar//com.comphenix.protocol.reflect.FuzzyReflection.getField(FuzzyReflection.java:352)
at ProtocolLib.jar//com.comphenix.protocol.injector.packet.PacketRegistry.createNewRegister(PacketRegistry.java:183)
at ProtocolLib.jar//com.comphenix.protocol.injector.packet.PacketRegistry.initialize(PacketRegistry.java:334)
at ProtocolLib.jar//com.comphenix.protocol.injector.packet.PacketRegistry.getClientPacketTypes(PacketRegistry.java:369)
at ProtocolLib.jar//com.comphenix.protocol.injector.PacketFilterManager.(PacketFilterManager.java:120)
at ProtocolLib.jar//com.comphenix.protocol.injector.PacketFilterBuilder.build(PacketFilterBuilder.java:121)
at ProtocolLib.jar//com.comphenix.protocol.ProtocolLib.onLoad(ProtocolLib.java:183)
at io.papermc.paper.plugin.storage.ServerPluginProviderStorage.processProvided(ServerPluginProviderStorage.java:59)
at io.papermc.paper.plugin.storage.ServerPluginProviderStorage.processProvided(ServerPluginProviderStorage.java:18)
at io.papermc.paper.plugin.storage.SimpleProviderStorage.enter(SimpleProviderStorage.java:39)
at io.papermc.paper.plugin.entrypoint.LaunchEntryPointHandler.enter(LaunchEntryPointHandler.java:40)
at org.bukkit.craftbukkit.CraftServer.loadPlugins(CraftServer.java:549)
at net.minecraft.server.dedicated.DedicatedServer.initServer(DedicatedServer.java:324)
at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1233)
at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:332)
at java.base/java.lang.Thread.run(Thread.java:1583)
Dump:
Parameters:
[NULL]
Sender:
com.comphenix.protocol.ProtocolLib@393689f4[
statistics=
packetTask=
tickCounter=0
configExpectedMod=-1
updater=com.comphenix.protocol.updater.SpigotUpdater@53a7278b
redirectHandler=
scheduler=com.comphenix.protocol.scheduler.DefaultScheduler@5ed36d22
commandProtocol=
commandPacket=
commandFilter=
packetLogging=
skipDisable=false
isEnabled=false
loader=io.papermc.paper.plugin.manager.DummyBukkitPluginLoader@b6146d7
server=CraftServer{serverName=Purpur,serverVersion=1.21-2256-de2e7a7,minecraftVersion=1.21}
file=plugins/.paper-remapped/ProtocolLib.jar
description=org.bukkit.plugin.PluginDescriptionFile@6e9177f0
pluginMeta=org.bukkit.plugin.PluginDescriptionFile@6e9177f0
dataFolder=plugins/ProtocolLib
classLoader=PluginClassLoader{plugin=ProtocolLib v5.2.0-SNAPSHOT-679, pluginEnabled=false, url=plugins/.paper-remapped/ProtocolLib.jar}
naggable=true
newConfig=YamlConfiguration[path='', root='YamlConfiguration']
configFile=plugins/ProtocolLib/config.yml
logger=com.destroystokyo.paper.utils.PaperPluginLogger@71fd70cf
lifecycleEventManager=io.papermc.paper.plugin.lifecycle.event.PaperLifecycleEventManager@5e02e324
allowsLifecycleRegistration=true
]
Version:
ProtocolLib v5.2.0-SNAPSHOT-679
Java Version:
21.0.3
Server:
1.21-2256-de2e7a7 (MC: 1.21)

Please read all the other issues here with the same error... the solution is there...

Should be fixed in the latest dev build so please use the latest dev build and try again: https://ci.dmulloy2.net/job/ProtocolLib/lastSuccessfulBuild/