hello

please I have a problem, yesterday I downloaded the GT and it worked perfectly. I joined many l4d2 servers and everything was okay until today.

Today, when I press on "join" at any server it shows to me a message:
"unable to Launch Requested Game as it was not Detected as installed.
Please Set up The requested game and try launching again."

I tried the solutions in the forum but nothing worked!

P.S: I've tried to add L4D2 manually but still the same error appear.
P.S.S: I've tried to run the game as administrator as well as the GT, still the same error
P.S.S: I tried to restart the laptop and delete the GT from regedit but still not work.

plz help