Have same issues here too.
Was working fine with the Game Pass version played about 25 hours loved it, I then bought "Forza Horizon 4 Ultimate Edition" during xmas sales and now the game won't launch.
For now I'm going to attempt a clean install of windows removing everything and re-download the game entirely. I'm assuming this should essentially reset the registry. I've also gone and deleted the WindowsApps folder myself to make sure it's completely wiped. (I'll report back if this fixes my issue.)
If this doesn't fix this issue then I'm just going to refund the game. This game came out in 2018, kinda ridiculous bugs like this are still in the game because of broken MS store and DRM implementation.