К сожалению, ошибка в данном коде не указана. Пожалуйста, уточните, что именно не работает, и приведите сообщение об ошибке, если оно есть. Без этого я не смогу помочь вам.
[20:49:37 ERROR]: null
org.bukkit.command.CommandException: Unhandled exception executing command 'buyer' in plugin sBuyer v1.0
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:47) ~[patched_1.16.5.jar:git-Paper-792]
at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:159) ~[patched_1.16.5.jar:git-Paper-792]
at org.bukkit.craftbukkit.v1_16_R3.CraftServer.dispatchCommand(CraftServer.java:826) ~[patched_1.16.5.jar:git-Paper-792]
at net.minecraft.server.v1_16_R3.PlayerConnection.handleCommand(PlayerConnection.java:2185) ~[patched_1.16.5.jar:git-Paper-792]
at net.minecraft.server.v1_16_R3.PlayerConnection.c(PlayerConnection.java:2000) ~[patched_1.16.5.jar:git-Paper-792]
at net.minecraft.server.v1_16_R3.PlayerConnection.a(PlayerConnection.java:1953) ~[patched_1.16.5.jar:git-Paper-792]
at net.minecraft.server.v1_16_R3.PacketPlayInChat.a(PacketPlayInChat.java:49) ~[patched_1.16.5.jar:git-Paper-792]
at net.minecraft.server.v1_16_R3.PacketPlayInChat.a(PacketPlayInChat.java:7) ~[patched_1.16.5.jar:git-Paper-792]
at net.minecraft.server.v1_16_R3.PlayerConnectionUtils.lambda$ensureMainThread$1(PlayerConnectionUtils.java:35) ~[patched_1.16.5.jar:git-Paper-792]
at net.minecraft.server.v1_16_R3.TickTask.run(SourceFile:18) ~[patched_1.16.5.jar:git-Paper-792]
at net.minecraft.server.v1_16_R3.IAsyncTaskHandler.executeTask(IAsyncTaskHandler.java:136) ~[patched_1.16.5.jar:git-Paper-792]
at net.minecraft.server.v1_16_R3.IAsyncTaskHandlerReentrant.executeTask(SourceFile:23) ~[patched_1.16.5.jar:git-Paper-792]
at net.minecraft.server.v1_16_R3.IAsyncTaskHandler.executeNext(IAsyncTaskHandler.java:109) ~[patched_1.16.5.jar:git-Paper-792]
at net.minecraft.server.v1_16_R3.MinecraftServer.bb(MinecraftServer.java:1271) ~[patched_1.16.5.jar:git-Paper-792]
at net.minecraft.server.v1_16_R3.MinecraftServer.executeNext(MinecraftServer.java:1264) ~[patched_1.16.5.jar:git-Paper-792]
at net.minecraft.server.v1_16_R3.IAsyncTaskHandler.executeAll(IAsyncTaskHandler.java:95) ~[patched_1.16.5.jar:git-Paper-792]
at net.minecraft.server.v1_16_R3.MinecraftServer.a(MinecraftServer.java:1400) ~[patched_1.16.5.jar:git-Paper-792]
at net.minecraft.server.v1_16_R3.MinecraftServer.w(MinecraftServer.java:1135) ~[patched_1.16.5.jar:git-Paper-792]
at net.minecraft.server.v1_16_R3.MinecraftServer.lambda$a$0(MinecraftServer.java:291) ~[patched_1.16.5.jar:git-Paper-792]
at java.lang.Thread.run(Thread.java:750) [?:1.8.0_322]
Caused by: java.lang.UnsupportedOperationException
at com.google.common.collect.ImmutableCollection.add(ImmutableCollection.java:218) ~[patched_1.16.5.jar:git-Paper-792]
at com.sbuyer.utils.BuyerManager.spawnBuyer(BuyerManager.java:35) ~[?:?]
at com.sbuyer.cmds.BuyerCMD.execute(BuyerCMD.java:22) ~[?:?]
at com.sbuyer.cmds.AbstractCMD.onCommand(AbstractCMD.java:27) ~[?:?]
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:45) ~[patched_1.16.5.jar:git-Paper-792]
... 19 more