Cannot change 360 online settings to enable me to purchase xbox 360 contents on my Xbox one

If you're on a child account, you'll have to make the purchase from a 360 content due to child restrictions.

otherwise, make sure you have either a credit card or paypal on file in order to make the purchase.

Hi there, you might want to just check your online privacy and security settings to see if purchasing is enabled as well as checking your content settings. Steps at the following link:

support.xbox.com/.../xbox-360-online-safety

Hey mate, I do have to admit that I didn't read through your whole post, it's in need of some paragraphs, but if you're having issues viewing and purchases certain content, try and change your online safety and privacy settings through the PC for both Xbox
360 and One. Follow the link for steps:
support.xbox.com/.../cannot-change-my-privacy-settings

Xbox One safety features enable you to control what kinds of content each of your family members can access on Xbox One. To set content restrictions, see How
a parent can change the privacy and online safety settings for a child account. For help changing these features, see Troubleshoot
changing online safety or privacy settings on Xbox One.

That is usually due to a setting in Privacy and Online Safety Settings: http://support.xbox.com/en-us/xbox-one/security/cannot-change-my-privacy-settings

Purchasing through Xbox One console or on the website Xbox.com is essentially the same method for Xbox 360 backwards compatible games, so make sure the settings for that are correct to allow you to make purchases.