If you cant access the campaign on PC Game pass, DONT PANIC!

Your progress is most likely fine, I dont know what kind of bug it is but Ive been playing no problems for two days and today it suddenly asked me to buy the campaign. I tried restarting and relogging a few times but nothing worked. I checked in Halo Waypoint and some guys had the same issue. They recommended reinstalling the whole game and so I did.

This should be checked or patched but dont worry about your progress. If youre like me and already have maxed out F.O.B.s you can breathe. Theyll still be there when you reinstall.