[16:24:37 INFO]: Error Cannot compile structure. Disabing compiler. (java.lang.reflect.InaccessibleObjectException: Unable to make protected final java.lang.Class java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int) throws java.lang.ClassFormatError accessible: module java.base does not "opens java.lang" to unnamed module @6385e0ca) occured in com.comphenix.protocol.reflect.compiler.BackgroundCompiler@1afb8f35.
[16:24:37 ERROR]: [ProtocolLib] INTERNAL ERROR: Cannot compile structure. Disabing compiler.
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.reflect.InaccessibleObjectException: Unable to make protected final java.lang.Class java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int) throws java.lang.ClassFormatError accessible: module java.base does not "opens java.lang" to unnamed module @6385e0ca
at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:357)
at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:297)
at java.base/java.lang.reflect.Method.checkCanSetAccessible(Method.java:199)
at java.base/java.lang.reflect.Method.setAccessible(Method.java:193)
at com.comphenix.protocol.reflect.compiler.StructureCompiler.generateClass(StructureCompiler.java:305)
at com.comphenix.protocol.reflect.compiler.StructureCompiler.compile(StructureCompiler.java:219)
at com.comphenix.protocol.reflect.compiler.BackgroundCompiler$2.call(BackgroundCompiler.java:202)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630)
at java.base/java.lang.Thread.run(Thread.java:831)
Dump:
Parameters:
com.comphenix.protocol.reflect.StructureModifier@7f8711de[
targetType=class net.minecraft.server.v1_16_R3.PacketPlayInFlying$PacketPlayInPositionLook
target=
converter=
fieldType=double
data=[public double net.minecraft.server.v1_16_R3.PacketPlayInFlying.x, public double net.minecraft.server.v1_16_R3.PacketPlayInFlying.y, public double net.minecraft.server.v1_16_R3.PacketPlayInFlying.z]
defaultFields={}
subtypeCache={}
customConvertHandling=false
useStructureCompiler=true
]
manager:
com.comphenix.protocol.injector.PacketFilterManager@6ab86d10[
unhookTask=com.comphenix.protocol.injector.DelayedSingleTask@4bfb8279
packetListeners=[PacketAdapter[plugin=MyCommand, sending=EMPTY_WHITELIST, receiving=ListeningWhitelist[priority=NORMAL, packets=[UPDATE_SIGN[class=PacketPlayInUpdateSign, id=41]], gamephase=PLAYING, options=[]]], PacketAdapter[plugin=TakaAntiCheat, sending=EMPTY_WHITELIST, receiving=ListeningWhitelist[priority=NORMAL, packets=[TAB_COMPLETE[class=PacketPlayInTabComplete, id=6]], gamephase=PLAYING, options=[]]], PacketAdapter[plugin=HolographicDisplays, sending=ListeningWhitelist[priority=NORMAL, packets=[REL_ENTITY_MOVE[class=PacketPlayOutRelEntityMove, id=41], REL_ENTITY_MOVE_LOOK[class=PacketPlayOutRelEntityMoveLook, id=42], SPAWN_ENTITY[class=PacketPlayOutSpawnEntity, id=0], ENTITY_METADATA[class=PacketPlayOutEntityMetadata, id=68], SPAWN_ENTITY_LIVING[class=PacketPlayOutSpawnEntityLiving, id=3]], gamephase=PLAYING, options=[]], receiving=EMPTY_WHITELIST], PacketAdapter[plugin=TakaAntiCheat, sending=EMPTY_WHITELIST, receiving=ListeningWhitelist[priority=NORMAL, packets=[BLOCK_PLACE[class=PacketPlayInBlockPlace, id=45]], gamephase=PLAYING, options=[]]], PacketAdapter[plugin=TakaAntiCheat, sending=EMPTY_WHITELIST, receiving=ListeningWhitelist[priority=NORMAL, packets=[CLIENT_COMMAND[class=PacketPlayInClientCommand, id=4]], gamephase=PLAYING, options=[]]], PacketAdapter[plugin=TakaAntiCheat, sending=ListeningWhitelist[priority=NORMAL, packets=[ENTITY_VELOCITY[class=PacketPlayOutEntityVelocity, id=70]], gamephase=PLAYING, options=[]], receiving=EMPTY_WHITELIST], PacketAdapter[plugin=TakaAntiCheat, sending=EMPTY_WHITELIST, receiving=ListeningWhitelist[priority=NORMAL, packets=[BLOCK_DIG[class=PacketPlayInBlockDig, id=26]], gamephase=PLAYING, options=[]]], PacketAdapter[plugin=TakaAntiCheat, sending=EMPTY_WHITELIST, receiving=ListeningWhitelist[priority=NORMAL, packets=[KEEP_ALIVE[class=PacketPlayInKeepAlive, id=15], POSITION_LOOK[class=PacketPlayInPositionLook, id=18], POSITION[class=PacketPlayInPosition, id=17]], gamephase=PLAYING, options=[]]], PacketAdapter[plugin=TakaAntiCheat, sending=ListeningWhitelist[priority=NORMAL, packets=[ENTITY_METADATA[class=PacketPlayOutEntityMetadata, id=68]], gamephase=PLAYING, options=[]], receiving=EMPTY_WHITELIST]]
packetInjector=com.comphenix.protocol.injector.netty.ProtocolInjector$5@5cedbff3
playerInjection=com.comphenix.protocol.injector.netty.ProtocolInjector$4@1b235c51
interceptWritePacket=com.comphenix.protocol.injector.packet.InterceptWritePacket@325a72c8
inputBufferedPackets=[]
recievedListeners=com.comphenix.protocol.injector.SortedPacketListenerList@25a5d6b0
sendingListeners=com.comphenix.protocol.injector.SortedPacketListenerList@1486e7d0
hasClosed=false
classLoader=PluginClassLoader{plugin=ProtocolLib v4.5.1, pluginEnabled=true, url=plugins/ProtocolLib (1).jar}
reporter=com.comphenix.protocol.ProtocolLib$1@15c6f0c5
server=CraftServer{serverName=Paper,serverVersion=git-Paper-776,minecraftVersion=1.16.5}
library=ProtocolLib v4.5.1
asyncFilterManager=com.comphenix.protocol.async.AsyncFilterManager@7ff054fc
knowsServerPackets=true
knowsClientPackets=true
phaseLoginCount=0
phasePlayingCount=9
packetCreation=false
spigotInjector=
nettyInjector=com.comphenix.protocol.injector.netty.ProtocolInjector@2f00da4a
pluginVerifier=com.comphenix.protocol.injector.PluginVerifier@2f4ebd33
hasRecycleDistance=true
minecraftVersion=(MC: 1.16.5)
loginPackets=com.comphenix.protocol.injector.LoginPackets@19c8b7c0
debug=false
]
Sender:
com.comphenix.protocol.reflect.compiler.BackgroundCompiler@1afb8f35[
listeners={com.comphenix.protocol.reflect.compiler.StructureCompiler$StructureKey@b8011c1b=[com.comphenix.protocol.reflect.compiler.BackgroundCompiler$1@3300d4aa], com.comphenix.protocol.reflect.compiler.StructureCompiler$StructureKey@cc64e2=[com.comphenix.protocol.injector.StructureCache$1@69fd50dc]}
listenerLock=java.lang.Object@35579c94
compiler=com.comphenix.protocol.reflect.compiler.StructureCompiler@73132f2c
enabled=false
shuttingDown=false
executor=java.util.concurrent.Executors$FinalizableDelegatedExecutorService@46830060
reporter=com.comphenix.protocol.ProtocolLib$1@15c6f0c5
disablePermGenFraction=0.65
]
Version:
ProtocolLib v4.5.1
Java Version:
16.0.1
Server:
git-Paper-776 (MC: 1.16.5)

I am also having this issue.
I am using java 16 and the problem goes away if i hop back to java 15

You need to read plugin description
There is a Version for Java 16 - Protocolib 4.6.1 Build 505
That is recomment to use when using java 16

If you might update to 1.17 check our your using the latest build 521 from jenkins for it.