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.

SteamUser_2004

FIXED: "CD Key Authentication Invalid" on internet servers (CSS)

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.

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.

Hope this saves someone the headache.