Do reinstall the game. Do this instead:
Update Steam and restart
Some modded servers require a specific file. Download the official Steamworks SDK Redistributable (v1.163 compatible) and place steam_api64.dll into your root assettocorsa folder, overwriting the old one. assetto corsa is obsolete v1163 is required fix hot
If you want to remove the warning immediately, you can trick the software by editing the game's internal version text file. Do reinstall the game