I am trying to load Forza Horizon 4 and keep getting error 0x903f9008.

If not already, since these were already suggested, try...

• Closing the game and reopening it.

• Power-cycle your Xbox One by holding the power button until it shuts off.

• Uninstalling and reinstalling the game.

Sometimes games have server issues, too. It's a possibility the problem could be on their end. I don't know, though.

As others have suggested, either power cycle the console or reinstall the game. You can also try clearing the cache on your xbox one.

Powercycle your xbox it usualy fixes the issue

Hello Sam,

Thanks for coming to the forums. Try and uninstall and reinstall the game. Also, what device are you playing the game on?

Thanks,

Brad

Try to power cycle the console, that often fixes a lot of issues with games not loading properly