Forza Horizon 5 needs to be installed at a Steam library location that is 59 characters or shorter.
Example valid Steam library location:
D:\Programs\Games\Steam\Racing\Online Multiplayer\Microsoft
Example invalid Steam library location:
D:\Entertainment\Games\Steam\Racing\OnlineMultiplayer\Microsoft
Steams guide on moving a games installation is detailed here: https://help.steampowered.com/en/faqs/view/4BD4-4528-6B2E-8327