why does it say "your account is unable to make this purchase when trying to buy an xbox...

what if I don't have an xbox 360?

You need to purchase 360 games from an Xbox 360 console.

I've had this issue before on the Xbox One. I'm assuming you're trying to get on of the Games with Gold titles or are trying to purchase some other backwards compatible content?

When it says that it usually means that the console isn't reading you have a valid billing account/address available for the Xbox 360, this can be confusing when most people assume that the console should just read their Xbox One billing account for BC content
purchases... but for some reason it both does and doesn't. The best steps to take would be by starting out on your Xbox One in the billing settings and making sure all the information there is correct. Then go to https://account.microsoft.com/billing/payments
and again make sure all the information there is correct. Try an make the purchase you were trying to make again or again from Xbox.com. If it still gives you that error, contact support and be as specific as possible about the nature of the purchase issue
and they should be able to sort it out.

That's what I did after I double checked my information was correct and notified support I was having issues with Xbox 360 content purchases on the Xbox One. For further noting, you don't need an Xbox 360 to make Xbox 360 content purchases so long as the
content is available through Xbox One Backwards Compatibility on the Xbox One or Microsoft Store.

You can contact support here.

https://support.microsoft.com/en-us/contactus

Then you're up the creek without a paddle or a boat. Unless you know someone who has a 360 and will allow you to add your account to their xbox and make the purchase.

1. This error message usually appeared on Xbox.com, however since the Store app and the Avatar app only load the website when a purchase is about to be made, the problem also transfers to the console.
2. I don't have an Xbox 360 console and don't know anyone who does. There is no way I could use its interface to bypass the error.
3. I am using a child account with all restrictions disabled. And I mean all. Both on Xbox One and Xbox 360. This has been checked many times and doesn't affect the website's behavior.
4. Cancelling my Gold subscription and then applying a new one doesn't help.
5. I can't claim the Gold titles with the parent account since it doesn't have a valid Gold subscription and I am not willing to pay twice