If you run into this error after decrypting your game files, there are a few user suggested fixes that may help you.
Your mileage may vary.
1. Restarting Steam after decrypting fixed this error for some.
2. Simply navigating to "\Steam\steamapps\common\left 4 dead 2" and running left4dead2.exe from there worked for others.
2. Simply navigating to "\Steam\steamapps\common\left 4 dead 2" and running left4dead2.exe from there worked for others.
Your mileage may vary.