hmm yeah that's not the case for me — i only see "forza horizon 4" and "forza horizon 4 ultimate edition unlock key" in the games list, both of which give the same error. seems like that code can apply to a zillion different scenarios, which isn't helpful...
I had this issue when trying to preload the Ultimate edition last week. It would download fine on my work PC but not on my main home PC. I scoured the web for a solution and trying a few things I ended up getting it to start downloading by going into the Microsoft Store app settings in "Offline Permissions".
After toggling the setting above to "On" (and possibly rebooting, can't remember) my download started.