Hello. My Minecraft version is 1.15.2, I'm using Paper v184. I tried to delete all plugins except LuckPerms to make sure there is no conflicts. There's a group "admin", I tried to add prefix like so:

/lp group admin meta addprefix 100 "Admin"

I checked prefix:

/lp group admin meta info

It shows that prefix is there. Then I checked myself:

/lp user MyName parent info

It shows that I'm in the group "admin" and it's my only group. But there is no prefix in chat. Same thing with suffixes.

You need a chat formatting plugin + vault if you want to display the prefixes in chat.

https://github.com/lucko/LuckPerms/wiki/FAQ

Thanks. Now it works.