Forza Horizon 4 all of a sudden needs Xbox game pass

Make sure the game purchase is showing up in your order history. If it is I would suggest uninstalling and re-downloading.

Make sure you have purchased the game under the account you are using and you didn't made use of it through xbox pass or gamesharing. Try to delete the game and install it directly from the marketplace, if you think the purchase was made under your account.
If you still have issues contact xbox support: http://support.xbox.com/contact-us

Thank you for the suggestion, but this didn’t work. I als never had Xbox game pass and never used gamesharing.

Hello, do you already performed a console power cycle? this will fix most of the times this kind of issues, here are the instructions in the following link:

https://support.xbox.com/en-US/xbox-one/console/power-cycle-console

Let us know if these sugestions fix your issue, or if you need more assistance?

HI Nico,

It seems like the account could be having some entitlement issues, to best resolve this, please use the
Contact Us portal to speak with an agent who will be able to take a look at your account and hopefully get you back in the game.

I hope this helps!