[22:06:04] [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.IllegalStateException: Unable to access field private final int net.minecraft.server.v1_12_R1.EnumProtocol.g
at com.comphenix.protocol.reflect.accessors.MethodHandleHelper.getFieldAccessor(MethodHandleHelper.java:119)
at com.comphenix.protocol.reflect.accessors.Accessors.getFieldAccessor(Accessors.java:84)
at com.comphenix.protocol.reflect.StructureModifier$$Lambda$209/0x0000000000000000.apply(Unknown Source)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:175)
at java.util.Iterator.forEachRemaining(Iterator.java:116)
at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:497)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:487)
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:241)
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:566)
at com.comphenix.protocol.reflect.StructureModifier.getFields(StructureModifier.java:183)
at com.comphenix.protocol.reflect.StructureModifier.(StructureModifier.java:116)
at com.comphenix.protocol.reflect.StructureModifier.(StructureModifier.java:101)
at com.comphenix.protocol.injector.packet.PacketRegistry.createOldRegister(PacketRegistry.java:115)
at com.comphenix.protocol.injector.packet.PacketRegistry.initialize(PacketRegistry.java:294)
at com.comphenix.protocol.injector.packet.PacketRegistry.getClientPacketTypes(PacketRegistry.java:327)
at com.comphenix.protocol.injector.PacketFilterManager.(PacketFilterManager.java:120)
at com.comphenix.protocol.injector.PacketFilterBuilder.build(PacketFilterBuilder.java:121)
at com.comphenix.protocol.ProtocolLib.onLoad(ProtocolLib.java:183)
at org.bukkit.craftbukkit.v1_12_R1.CraftServer.loadPlugins(CraftServer.java:323)
at net.minecraft.server.v1_12_R1.DedicatedServer.init(DedicatedServer.java:222)
at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:616)
at java.lang.Thread.run(Thread.java:823)
Caused by: java.lang.IllegalAccessException: illegal setter on final field
at java.lang.invoke.MethodHandles$Lookup.findSetter(MethodHandles.java:808)
at com.comphenix.protocol.reflect.accessors.MethodHandleHelper.getFieldAccessor(MethodHandleHelper.java:96)
... 24 more
Dump:
Parameters:
[NULL]
Sender:
com.comphenix.protocol.ProtocolLib@b444fe80[
statistics=
packetTask=
tickCounter=0
configExpectedMod=-1
updater=com.comphenix.protocol.updater.SpigotUpdater@47b0cc35
redirectHandler=
scheduler=com.comphenix.protocol.scheduler.DefaultScheduler@ad029e5f
commandProtocol=
commandPacket=
commandFilter=
packetLogging=
skipDisable=false
isEnabled=false
loader=top.iseason.bukkit.model.PLoader@a9a17831
server=CraftServer{serverName=Paper,serverVersion=git-Paper-1620,minecraftVersion=1.12.2}
file=plugins\ProtocolLib.jar
description=org.bukkit.plugin.PluginDescriptionFile@d13f1f71
dataFolder=plugins\ProtocolLib
classLoader=org.bukkit.plugin.java.PluginClassLoader@3d9404cb
naggable=true
newConfig=YamlConfiguration[path='', root='YamlConfiguration']
configFile=plugins\ProtocolLib\config.yml
logger=com.destroystokyo.paper.utils.PaperPluginLogger@5aaf2d
]
Version:
ProtocolLib v5.1.0
Java Version:
1.8.0_282
Server:
git-Paper-1620 (MC: 1.12.2)

You prolly need version 4.4.0 for MC 1.12.2 https://github.com/dmulloy2/ProtocolLib/releases/download/4.4.0/ProtocolLib.jar

You prolly need version 4.4.0 for MC 1.12.2 https://github.com/dmulloy2/ProtocolLib/releases/download/4.4.0/ProtocolLib.jar

thank you！