Can't level up

[Legion88]

Check on Mogstation, which kind of expansion you bought and registered.
Seems like you have changed the plattform you are playing on and only registered the basegame ARR to it and the expansions to the old plattform.

You have to buy and register the game for every plattform you want to play on (PS4 and PC) seperately.
If you don't do this you can't, for example, level up on a platform you haven't registered Heavensward or Stormblood past 50.

As example:
You played on PS4 and registered ARR and Heavensward for this platform and you have registered only ARR on PC.
With this setuup you can only level up past 50 and gain experience past 50 if you play on PS4, if you would play on PC you wouldn't be able to log in to your character if you are only in a Heavensward area.