You can not claim this city because you do not own some of the content used to create it

I don't know how to fix this problem, but it is because the old owner of the city had the Collector's Edition of the game.

So the city has probably buildings from the extra content of that Edition. That's why uou can't play that city.

I have this problem in a region I set up. Next week I'll try to let that player (he has now an other city in the region) completely empty the city. Maybe that will solve this problem a bit.

A better solution would be an option to demolisch / remove an abandoned city...