If you are unable to launch the game due to Error 51, here is how you can fix it.
This is a Steam error that can occur due to one of many background programs. Please search for, and disable programs that could potentially interfere with Steam. Most notably:
Programs for recording and screen capturing.
Some Antivirus software can interfere with the Steam client and games and prevent some from running. You may need to add exceptions for Steam and its games in your antivirus configuration. Detailed instructions for exceptions can be found in your software(s) help centre(s).
It is also a good idea to verify the integrity of your game files:
Via Steam open your game library.
Right click on the game title and select "Properties".
Select the "LOCAL FILES" tab and click on "VERIFY INTEGRITY OF GAME FILES...".