This feature is blocked per your xbox live parental controls.

Same can`t play online either....

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

Hello farlig!

Thanks for reaching out to Xbox forums support. I'm sorry to hear about the parental controls error message that you're receiving on your Xbox console when trying to play Forza Horizon 4. I've actually come across this error message before and it usually
just corrects itself with a little time. I believe it has to do with the Forza developer server side of things, considering all your other games probably work just fine. If you want, you can try uninstalling / reinstalling the game. That might help. Try it
out and let us know how it goes!

(Helpful Link)

UNINSTALL | REINSTALL XBOX GAME


Let us know how it goes~

Felix

Xbox forums support

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.