I had the same issue as you, it's because you're not properly mounting CS:S. In your `drive:\gmod\garrysmod\cfg\mount.cfg` file it should look something like this:

"mountcfg"
{
// "cstrike" "C:\steamcmd\steamapps\common\Counter-Strike Source Dedicated Server\cstrike"
"cstrike" "D:\cssds\cstrike"
// "tf" "C:\mytf2server\tf"
}

Also, make sure that it's pointing to the cstrike folder inside of the cs:s install directory.

Thank you, dude! Love you