If you're receiving this error message, it means that you currently do not have permission to play FINAL FANTASY XIV on the SQUARE ENIX Account you are currently logged in with.
Typically, this occurs when a pre-order or Early Access period ends. Please double-check that you have already registered the expansion you are attempting to access to the relevant SQUARE ENIX Account. You can do so by logging into the Mog Station at the following link:
https://www.mogstation.com