Many mods require the 7-Zip plugin within Content Manager to unpack correctly. If a link was provided in the error message, it is likely a direct download for the specific CSP version or a missing car pack needed for that server.

If you aren’t using it yet, download Content Manager. It is the essential "launcher" for modern Assetto Corsa. Update CSP: Open Content Manager. Go to Settings > Custom Shaders Patch > About & Updates .

You are genuinely running an older version of the game (common if you haven’t logged into Steam in years or are using an offline backup).