There are several reasons this error appeared, ranging from legitimate technical issues to piracy:
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. There are several reasons this error appeared, ranging
I had an old key from a Half-Life 2 box. I activated it in Steam, but CS:S still said "invalid." Turns out, that key was for the original 2004 CSS (pre-Orange Box) which is no longer supported for internet play. I had to: There are several reasons this error appeared, ranging
: Deleting the local cache forces Steam to re-authenticate your account credentials. Completely shut down Steam. There are several reasons this error appeared, ranging