Just watch this video.
https://youtu.be/Ql230AsNzFQ

You need a chat plugin that supports Pex

EDIT: You have Chat Plugin Pex but You were doing the permissionsEX Sorry but you are using the Wrong Command You were setting your prefix in world &4

Command from video: pex group ejer prefix &4[Ejer] &4

Right Command: pex group ejer prefix "&4[Ejer] &4"

Sorry, I keep editing the question. I keep making typos.

Unfortunately PermissionsEx is currently unmaintained and no further updates are planned. Because of this, all issues are being closed. If you are looking for an actively maintained permissions plugin, take a look at LuckPerms. It is compatible with major server platforms and can import data from PEX.