Nevermind. If you guys have the same problem I fixed it this way:
-Open the directory where the game is installed (the folder where rev.ini file is) then go to the folder left4dead2, there you will find a file called steam.inf. Open steam.inf with notepad and there change the version for PatchVersion to 2.1.4.2. Save the file and start the game and you should be able to join the server.
Hope it worked XD