Make sure your Steam account actually owns the modern version of CS:S, opt out of betas, verify files, and avoid cracked/old servers.
Hope this saves someone the headache.
SteamUser_2004
If you bought a key from a third-party site (G2A, Kinguin, etc.) for $2, it’s almost certainly a stolen or region-locked key. Steam will reject it for internet play. Refund it if you can. Make sure your Steam account actually owns the
Your Steam account does not own a valid Counter-Strike: Source license for the version you’re trying to play. This usually happens with old retail (disc) keys or third-party keys. Steam will reject it for internet play
I know this is an old error, but I just ran into it after digging out my old copy of CS:S, and it took me an hour to fix. If you’re getting when trying to join internet servers (but LAN works fine), here’s what’s actually happening and how to solve it. This usually happens with old retail (disc) keys