What Cisyouc said, only if you are trying to run a server.... If you are simply trying to play, then it will depend on the server, whether they have it enabled... Another cause could be if you are patched TOO old for any of the servers that you see.. If you are trying to play on anything BEFORE 5.7 then you will have had to patch to a certain date (which may or may not include GOD)... If you are trying to play on 5.8 servers, then simply make sure you are patched up to live (using the advanced button make certain that everything except Omens is checked and repatch).... Hope this helps some..
