Is this a region you were playing before you reinstalled? If so, you might have missed installing an add-on from your account; double-check all the SimCity things in Origin just in case. But, I've heard of people abandoning their own regions and then not able to claim them again, so it's hard to tell.
If you're trying to take over an abandoned region, this is one of the most annoying things about the multiplayer mode. There are promo add-ons that are no longer available, special pre-order bonuses, and country-specific dlcs that can't be obtained any more. I also suspect that they coded the check wrong, looking for a specific dlc rather than the actual add-on. (For instance, my friend and I have exactly the same downloads - he bought the digital deluxe version, I have all the dlc's that are in his version, but mine were purchased at launch on CD. I can't take over anything that he abandons)
I wrote a post a while ago outlining the problem and asking EA to make them all available: https://answers.ea.com/t5/SimCity-2013/Request-to-EA-Make-Promo-AddOns-Available/td-p/6652082. The post didn't get any traction, and I don't see anything changing, so there will continue to be abandoned cities that no one can claim cluttering up the servers. Wish I had better news.
