This is due to the game not connecting to the Counter-Strike servers. To fix this, follow those steps
Open up your normal CS:GO and keep it running in the background. (I suggest to limit the FPS in the menu by using fps_max 1 & fps_max_menu 1 in the console to limit CS:GO using your resource in the background)
Open up now CS2 and wait a little. It should connect you now to the Counter-Strike network in CS2
I get "Unable to establish a connection with the gameserver." when I try to join a server
You probably tried to join a server that is offline at the moment. Try to join a different server.