"You can't play multiplayer games because of how your account is set up."

Did you enter your correct birthdate then, when you created the account?

Also: Do you properly see your online and privacy settings, when you go to http://account.xbox.com/settings, or do you get a page explaining
how to add your account to a parent account?

So how exactly doesn't "it" work?

Hi Tanvir!

On which platform do you play the FH4 game? Steam platform or Xbox/Windows platform?

On the privacy settings here https://account.xbox.com/Settings, does it show a different gamertag from your Xbox app? If so, check the email address of your account by going to https://account.microsoft.com > Your info. Use that email address to sign in to your Xbox app. Sign out from Xbox app and sign in your correct account.

What exactly doesn't work?

If you go to http://account.xbox.com/settings and see a page explaining how to add a child account to a parent account, you will need the
help of your parents with an adult Microsoft account.

If not already done so, your parent has to go to
https://account.microsoft.com/family and add your account to their family.

Then your parent has to go to http://account.xbox.com/settings, sign in with their parent
account and make the necessary changes on your child account.

After your parents have set permissions on your account, you need to sign out and back in on the device or app you're trying to use.

Perfect! I am glad that everything works fine now. Thank you for your patience and time here in the community.

Take care and enjoy!

I use windows, and yes it shows a different one. On https://account.xbox.com/Settings it says poliseal610 and xbox app says RainbowTrain715. I signed into the account on xbox app with the email on "Your Info" everything works now thank you very much.