Hi iBeaver,
this seems to be a problem more related to Steam than the game itself. Please check this forum post:
http://gsngaming.com/topic/3254-stea...error-code-51/
They suggest renaming your folder with Steam games (steamapps to steamapps_old) and then re-downloading the game. You can also contact our tech support at support [at] 1cpublishing.eu, but as I mentioned, this is more of a Steam-client related issue, since users are reporting that many other games cannot be launched with the same error code 51.
Jan