could i please have permission to use rcon menu, every time i try to use it now it says "only certain players are allowed to use rcon menu". How do i get permission?

sv_rconusers

That's for server admins. You gotta ask the owner of that server for the permissions. Has nothing to do with Funky Fortnite.

so how do i figure out who is the server admin, and how do i communicate with them?

You've to contact them on their server community forums about that, if they do host a forum in the first place.