won't let me buy the overlord 2 game on an xbox one

Are you trying to buy the game, or claim it as a
Games with Gold title?

Also, if you don't mind me asking, what region are you in? Because sometimes Games with Gold titles can vary depending from region to region.

Lastly, you need an active payment option, like a credit card, to purchase Xbox 360 games on an Xbox One.

I'm having the same issue only it's not a regional issue, mine is a Won't Process issue. I just get a spinning Xbox circle when I click confirm from marketplace.xbox.com. I have lost nemours games because of this and I'm starting to wonder If I should
even continue with Xbox anymore. I'm honestly getting sick of all the hoops I have to jump through with this service and constantly updating my console. I don't like being redirected away from my xbox console to buy something. I used to be able to buy games
directly through the console or app. Payment info is up to date and tried 3 different browsers, same issue.

No, i just stopped by Gamestop and bought the game there,

Hello!

It sounds like the payment is getting declined. I would contact your bank/credit card company to see if they are blocking it, and if so, let them know this is an authorized purchase. Most likely it will work the next time you try.

There was a Xbox Live Service alert yesterday and content purchases may have been affected. Have you tried to purchase the game again today?