: Cracked or non-Steam versions of the game may have modified or corrupted DLL files that do not match official server environments.
The error is literal. When you attempt to connect to a CS 1.6 server, the server performs a checksum (a digital fingerprint) on its own version of client.dll . Your client sends a copy of its own client.dll checksum. If they do not match, the server assumes you are running an incompatible—or potentially malicious—version of the file. cs 1.6 client.dll differs from server
You can allow players with slightly different files (common in the non-Steam community) to join by adding a simple command to your server configuration. : Cracked or non-Steam versions of the game