[21:02:13] [Netty Epoll Server IO #3/WARN]: [OldCombatMechanics] Error in a packet listener (send).
java.lang.NullPointerException: Cannot invoke "java.util.Optional.isPresent()" because "structOptional" is null
at kernitus.plugin.OldCombatMechanics.utilities.packet.team.V17TeamPacket$PacketAccess.lambda$setCollisionRule$4(V17TeamPacket.java:121) ~[OldCombatMechanics.jar:?]
at kernitus.plugin.OldCombatMechanics.utilities.reflection.Reflector.getUnchecked(Reflector.java:294) ~[OldCombatMechanics.jar:?]
at kernitus.plugin.OldCombatMechanics.utilities.packet.team.V17TeamPacket$PacketAccess.setCollisionRule(V17TeamPacket.java:117) ~[OldCombatMechanics.jar:?]
at kernitus.plugin.OldCombatMechanics.utilities.packet.team.V17TeamPacket.withCollisionRule(V17TeamPacket.java:32) ~[OldCombatMechanics.jar:?]
at kernitus.plugin.OldCombatMechanics.module.ModulePlayerCollisions$CollisionPacketListener.handlePacket(ModulePlayerCollisions.java:172) ~[OldCombatMechanics.jar:?]
at kernitus.plugin.OldCombatMechanics.module.ModulePlayerCollisions$CollisionPacketListener.onPacketSend(ModulePlayerCollisions.java:141) ~[OldCombatMechanics.jar:?]
at kernitus.plugin.OldCombatMechanics.utilities.packet.mitm.PacketInjector.write(PacketInjector.java:187) ~[OldCombatMechanics.jar:?]
at io.netty.channel.AbstractChannelHandlerContext.invokeWrite0(AbstractChannelHandlerContext.java:879) ~[netty-transport-4.1.97.Final.jar:4.1.97.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeWrite(AbstractChannelHandlerContext.java:863) ~[netty-transport-4.1.97.Final.jar:4.1.97.Final]
at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:968) ~[netty-transport-4.1.97.Final.jar:4.1.97.Final]
at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:856) ~[netty-transport-4.1.97.Final.jar:4.1.97.Final]
at io.netty.channel.DefaultChannelPipeline.write(DefaultChannelPipeline.java:1015) ~[netty-transport-4.1.97.Final.jar:4.1.97.Final]
at io.netty.channel.AbstractChannel.write(AbstractChannel.java:301) ~[netty-transport-4.1.97.Final.jar:4.1.97.Final]
at com.comphenix.protocol.injector.netty.channel.NettyChannelProxy.write(NettyChannelProxy.java:210) ~[ProtocolLib.jar:?]
at com.comphenix.protocol.injector.netty.channel.NettyChannelProxy.write(NettyChannelProxy.java:199) ~[ProtocolLib.jar:?]
at net.minecraft.network.Connection.doSendPacket(Connection.java:449) ~[?:?]
at net.minecraft.network.Connection.lambda$sendPacket$10(Connection.java:434) ~[?:?]
at com.comphenix.protocol.injector.netty.channel.NettyEventLoopProxy.lambda$proxyRunnable$2(NettyEventLoopProxy.java:48) ~[ProtocolLib.jar:?]
at io.netty.util.concurrent.AbstractEventExecutor.runTask(AbstractEventExecutor.java:174) ~[netty-common-4.1.97.Final.jar:4.1.97.Final]
at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:167) ~[netty-common-4.1.97.Final.jar:4.1.97.Final]
at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:470) ~[netty-common-4.1.97.Final.jar:4.1.97.Final]
at io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:413) ~[netty-transport-classes-epoll-4.1.97.Final.jar:4.1.97.Final]
at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997) ~[netty-common-4.1.97.Final.jar:4.1.97.Final]
at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) ~[netty-common-4.1.97.Final.jar:4.1.97.Final]
at java.lang.Thread.run(Thread.java:1583) ~[?:?]

paper 1.20.4
latest plugin
just fight a player and it fills console

Please fill in the issue template. "latest plugin" doesn't mean anything.

ffs you know what exactly latest is XD OldCombatMechanics - Disable 1.9 hit cooldown 2.0.1 better?

Well no, I don't know what "latest" means, as this changes quite frequently and there are multiple places where you might've gotten the plugin. In fact, the version you're using is not the latest, because this issue was already fixed in the newer versions you can find on Jenkins.