I have the Plex Pass but when I try to add users a message appears saying I need the Plex Pass, so I am unable to add users. So now I’m confused!!
Are you sure about that?
The Plex account which you are using to write here has no Plex Pass, as far as I can see.
If you have a second plex account, the server must be signed in to this, of course.
But I’d double check first, if you really bought a Plex Pass and not just paid the one-time unlock fee for one of the mobile Plex apps.