Some third party aircraft require authorization codes. Depending on where & when you see this it could be from X-Plane's DRM or a third party DRM.
If it is at the very launch of X-Plane, please enter your product key in the authorization window. It should remember it after that. X-Plane should authorize in the background any time you have an internet connection. If you do not have an internet connection, it should still work for about 2 weeks before it needs to connect to the server to re-authorize.
If this is not working for you, do you see a warning about not being able to write to preferences? Attach a copy of the log.txt for further investigation.