How to enable commands on a server?

[TallyTuatara]

I've already op'd myself in the console section but when I try to run commands in game it says its unknown. I've already op'd myself again and it says I'm already op'd so I'm not really sure why commands aren't working. If anyone has an answer that would be great. 

[PrecedingMC]

What command are you trying to use, and what plugins are on your server? Ensure you're typing your username correctly as well.

[1m2a4x]

do /pl is any of the plugins outdated