Propper Error (Portal 2)

Hi there, I get an error when i try to compile my vmf with propper

MountFilesystem( 621 ) failed: SteamMountFilesystem(8232,621,0x1e3ffb68=,0x1e3f
fa58) failed with error 5: No CS online with App 621 Ver 22

http://imageshack.us/a/img407/4046/cpx0.png

So yeah Idk what to do now.