Bukkit

While I was setting up my server, I noticed the difficulty was at peaceful, so I restarted the server to make it normal. When I got back on every /lobbyapi command including /lobby and /hub results in the message "An internal error occurred while attempting to perform this command" and I would prefer to not have to reinstall again after all the time I put into customizing each world and the hub menu. Heres what I see in my console: 

[23:26:48] [Server thread/INFO]: Lunaa43 issued server command: /hub
[23:26:48] [Server thread/ERROR]: null
org.bukkit.command.CommandException: Cannot execute command 'hub' in plugin LobbyAPI v3.7.103 - plugin is disabled.
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:37) ~[patched_1.15.2.jar:git-Paper-93]
at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:159) ~[patched_1.15.2.jar:git-Paper-93]
at org.bukkit.craftbukkit.v1_15_R1.CraftServer.dispatchCommand(CraftServer.java:742) ~[patched_1.15.2.jar:git-Paper-93]
at net.minecraft.server.v1_15_R1.PlayerConnection.handleCommand(PlayerConnection.java:1820) ~[patched_1.15.2.jar:git-Paper-93]
at net.minecraft.server.v1_15_R1.PlayerConnection.a(PlayerConnection.java:1628) ~[patched_1.15.2.jar:git-Paper-93]
at net.minecraft.server.v1_15_R1.PacketPlayInChat.a(PacketPlayInChat.java:47) ~[patched_1.15.2.jar:git-Paper-93]
at net.minecraft.server.v1_15_R1.PacketPlayInChat.a(PacketPlayInChat.java:5) ~[patched_1.15.2.jar:git-Paper-93]
at net.minecraft.server.v1_15_R1.PlayerConnectionUtils.lambda$ensureMainThread$0(PlayerConnectionUtils.java:23) ~[patched_1.15.2.jar:git-Paper-93]
at net.minecraft.server.v1_15_R1.TickTask.run(SourceFile:18) ~[patched_1.15.2.jar:git-Paper-93]
at net.minecraft.server.v1_15_R1.IAsyncTaskHandler.executeTask(IAsyncTaskHandler.java:136) ~[patched_1.15.2.jar:git-Paper-93]
at net.minecraft.server.v1_15_R1.IAsyncTaskHandlerReentrant.executeTask(SourceFile:23) ~[patched_1.15.2.jar:git-Paper-93]
at net.minecraft.server.v1_15_R1.IAsyncTaskHandler.executeNext(IAsyncTaskHandler.java:109) ~[patched_1.15.2.jar:git-Paper-93]
at net.minecraft.server.v1_15_R1.MinecraftServer.ba(MinecraftServer.java:1038) ~[patched_1.15.2.jar:git-Paper-93]
at net.minecraft.server.v1_15_R1.MinecraftServer.executeNext(MinecraftServer.java:1031) ~[patched_1.15.2.jar:git-Paper-93]
at net.minecraft.server.v1_15_R1.IAsyncTaskHandler.awaitTasks(IAsyncTaskHandler.java:119) ~[patched_1.15.2.jar:git-Paper-93]
at net.minecraft.server.v1_15_R1.MinecraftServer.sleepForTick(MinecraftServer.java:1015) ~[patched_1.15.2.jar:git-Paper-93]
at net.minecraft.server.v1_15_R1.MinecraftServer.run(MinecraftServer.java:938) ~[patched_1.15.2.jar:git-Paper-93]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_131]

I can see that it says the plugin becomes disabled, but I don't know how to reenable it and stop this error in the future.

That error simply states that the plugin was disabled. Could you look at the start up messages to see when LobbyAPI is disabled, and whether there are any errors at that time?

As for the difficulty issue, this will be addressed. By default, worlds use the original world's difficulty ("world"). The ability to set different difficulties per world will be added in the next update.