If you’re hosting a server, clients will see "server unavailable" if your ports aren’t open.

Assetto Corsa requires a live connection to your Steam profile to verify game ownership when entering a server. Always launch the official Steam client and log in before opening Content Manager. 5. Address Server-Side Hosting Issues

Furthermore, some community-run servers use Discord for whitelisting. To prevent griefing, they require you to link your Steam account to their Discord server before you're allowed to join their Assetto Corsa server. If you haven't completed this verification step, you will likely get a connection error.