Hey there
@c :D,
The command /pl is disabled because we do not share the plugins we use or information about them to the public. ManaCube uses various amounts of plugins, some custom made, some not, so we keep them private so that people aren't searching for exploits or taking them to create their own version of our server.
If you ever need help with commands or are unsure how to use certain ones, you can always ask in chat or if that doesn't work, ask a staff member via /helpop or /msg in-game. We are always here to help!