Xbox One won't let me purchase Xbox 360 games

Did this error just start or has this been going on for a while? Have you made purchases before for 360 games this way, or is this the first time you've tried it? If it worked before and suddenly is doing this now, it could possibly be a bug and I'd say
try again later. But if this is the first time you're trying it and you're getting this, then yes, sounds like an account issue with the settings and the way the account might have been set up.

This error means that the Microsoft account that you're trying to use to make the purchase doesn't have the ability to make the purchase.

Child and teen accounts are prevented from making purchases in the Xbox 360 Marketplace. Changing your Microsoft account age or your purchase settings won't resolve this issue.

You will have to purchase it from Xbox 360, there is no other option for you, sorry.

two things first your parental account must be gold

Not home gold

Second if you made it a parental account by changing the age of your account that doesn't work

Once set the age of an account is set in stone till it hits +18

Info on common questions here:
http://bit.ly/1pK1QXq