Can't buy games from the xbox web store

I have had this same issue for some time now as well, so what I have been able to gather is that the payment info doesn't match up between the two xbox stores, which is the 360 and One store. Therefore, you would need to add in your payment info directly
on your Xbox 360 in order to be able to make purchases on there.

This is what I'm trying out as soon as I can make my 360 wifi capable.

Best of luck and let us know if this solution works.

Please power cycle your console. Link. As described on the bottom of the page. If that not works try removing and then add your account again.
Wich is your payment method?

Maybe try to purchase on a different device? Or maybe it's an internet issue?

This is happening only in the console or is happening also by browser or through the App in Windows10 or Mobile?

I'm trying to buy a game from the web store but when I press confirm it just lodes endlessly, what do I need to do to fix this?

Try the app if you can't from your console at the moment. Could be a temporary bug.