My Account wont let me buy Xbox 360 games on my Xbox One

So I believe I made a mistake at the very beginning when I and my sibling had got an Xbox One.

I was the one above the age of 18 at the time so I was supposed to be the adult in the Xbox family, however, this got mixed up and I turned out to be the child for all this time this did make much of a difference as I just changed my account settings from
his account.

Not so long ago backward compatibility came up and this is when the issue really came up. When I tried to buy any 360 games on my account for example. skate 3 and red dead redemption I wasn't able to and this message came up when I tried to purchase it after
I pressed continue it said "sorry your account is unable to make this type of purchase on xbox.com. please complete this transaction from your Xbox.". however, I didn't want to buy myself and my brother an Xbox 360 so we could buy games on it to play it on
Xbox one there would be no point for the backward compatibility and I would be throwing money away as I sold my Xbox 360 a while now. So I tried it on my brothers account and it turned out you could the game on Xbox One, however only on his account now temporarily I
purchased the games on his account so I could still play it on the Xbox but recently I got a separate Xbox so we could play a few games multiplayer which don't have split screen and this issue came up again where I couldn't buy the game on the Xbox some games
I was able to purchase through xbox.com however not all the games allowed that.

Now if anyone knows a solution to this I would be so gratefull.

Thank You

:)

 

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