New store won’t let me buy things.

This also happens in the normal store also.

well, when I buy a new code, it won’t let me use that code

Buy Sonic Adventure 2 for Xbox 360

Ok so you dont say on which device you have tried to purchase it on... I assume you are trying on Xbox One?? If so hit the link above and try to purchase it there.. it directs you to the Xbox store via Browser. You can do this from a PC, phone or tablet...
and actually the Edge App on your console.. not via the inbuilt store accessed from your Dashboard

Some Xbox 360 Back compatible Titles cannot be purchased via your One console for various reasons.. this issue could be linked to that.

The way is to order the older games is via a browser if you havent access to a X360.. the purchase should then show up within games ready to install on your xbox one.. Let me know if you get any joy trying that way

Thats actually a good point, ive never gone straight to the top and asked xbox about it. I’ll hit up xbox support and see what they can do about it, ill keep you guys posted. Thanks for taking the time to help me out

You should definitely see if support can help you out on this, considering that you've tried multiple times.