I can't buy any Xbox 360 games from either my web browser on Windows and from my Xbox One

You can contact support at https://support.xbox.com/contact-us

From there you will need to request a call back from support.

It may be your method of payment. In order to purchase 360 games on your Xbox One, you may need a credit card on file. Support should be able to help you complete your purchase.

Best wishes,

Cowgirl

are you just doing this on your Xbox if so try try to do it on your PC instead cuz I've had that issue before and going to like my PC or my phone and and all and doing it through that way sometimes helps if that don't solve the issue I would call
Microsoft then

I have tried on my PC and also on my Xbox One. Everytime I try on my PC AND on my Xbox one.

This is what I get from my PC:

Thanks.

Where is the number to call Microsoft?

are you just doing this on your Xbox if so try try to do it on your PC instead cuz I've had that issue before and going to like my PC or my phone and and all and doing it through that way sometimes helps if that don't solve the issue I would call Microsoft
then

Yeah i know about the xbox/windows version thankfully I have not seen any windows versions on the web browser of the xbox store. I think they have the microsoft store and xbox version of the store separate now. Just wanted to make sure i can still download onto my console if i buy it from my phones web browser