If the game shows in your library, but you are unable to install the game and see that the game errors, try this.
ERROR (Code: 0x803FB107), (Code: 0x80073D12)
BE CAREFUL. For whatever reason, you may have to download the regular version of the game and then the add-ons
I was getting an error in Microsoft windows download section stating, " cannot install because game title hasn’t installed. in short, it was trying to install the add-ons and dlc BEFORE the game. This would stop the game from installing all together.
What I did to rectify the issue and it worked.
Opening Microsoft store from Xbox app > Click the 3 horizontal (…) next to your profile icon > Choose Downloads and Updates > Click “Ready to Install” on left side of screen under (My Library) > Locate the FORZA HORIZON 4 GAME and click install.
Hope this helps fellow gamers who are having troubles installing on PC.