Error code 0x803F8001 usually indicates a licensing issue, which could be caused by a number of factors.
#Forza Horizon 5 is currently not available in your account. Make sure you are signed in to the Store
bright perch
Check your account status: Make sure that you're signed in to the Microsoft Store using the same account that you used to purchase the game. Sometimes, signing out and signing back in can help.
Verify game ownership: Go to your library in the Microsoft Store and make sure that Forza Horizon 5 is listed as "Owned." If it isn't, you may need to repurchase the game or contact Microsoft Support for assistance.
Check for updates: Make sure that Forza Horizon 5 is up to date by checking for any available updates in the Microsoft Store.
Reinstall the game: Uninstall the game and then reinstall it from your library in the Microsoft Store.
Contact Microsoft Support: If none of the above steps work, you may need to contact Microsoft Support for further assistance. They can help you troubleshoot the issue and get you back to playing Forza Horizon 5 as soon as possible.