This feature is blocked per your xbox live parental control settings

Same can`t play online either....

It just says "This feature is blocked per your Xbox live parental control settings.

Do you get any error code or message when trying to log in from the website?

Xbox One has age-appropriate controls and settings that you can adjust to suit your family’s needs. Access these settings from either Xbox.com or an Xbox One console—though you can only change content and app settings from the console.

From Xbox.com Change privacy and safety settings for you or a child

1. Sign in to the adult's Microsoft account at Xbox.com.
   If you’ve forgotten your account email address or password, see the
   
   Lost Account Solution or the
   Lost Password Solution.
   
2. Under My Xbox, select Profile.
3. Select Privacy settings or Online safety on Xbox One.
   Due to the secure nature of these settings, you may be asked to enter a security code that we’ll send to the alternate phone number or email address previously saved to your account.
   
4. Select the account you want to look at.
5. Review the settings currently in place and update those you want to change.
6. Save your changes on each tab before moving on to the next.

I have been looking at questions with the same issue as me and none of it makes sense to me, that link you sent also isnt much help as i have not spent much time in my Microsoft account or the settings or xbox live account.

Yeah, the information that M3tal Daz3 sent above should work. This is the link to the full page if you need it: https://privacy.microsoft.com/en-US/xbox-one-privacy-and-online-safety