Hi Philip,
I am not with Laminar Research; just a flight simmer.
Normally I would try and provide an answer but in this case I am offering some additional information that may help.
My presumption is that you have a disc version of X-Plane. If so then disc 1 has to be inserted into your disc drive at all times to run the software. If you don't you will always get the message you are having difficulty with. There is no product key for and will not run a disc version.
If I am incorrect and you are running a digital download version then you have to apply the product key provided and be connected to the internet at all times. My understanding is there is a "silent" check performed to ascertain the legality of your software.
If you don't insert the disc or apply the product key the software reverts to the demo mode which has a time and location restriction.
As a further sideline issue to run X-Plane you need to be running the PC(?) as the administrator.
Good luck.
Glenn