Every time I try to join an online game I get that error. I'm playing the demo but did buy the full version too. Anyway all I can see is black and the fire from the nuke someone just shot at me!

Its because the server is currently running a map you donot have.

If its a full version server, you need the full version to play.

If its a modded server you need both the full version and the map the modded server is running.