I think I've seen the same symptom once in one of my games. We found out, what the problem was and solved it as follows:
The game had originally been started with 5 human players. One dropped out after the first session of play, so we decided to go on with 4 people. So four of us joined the game, I launched it, and then we got the same error message as you got.
The solution was: I found a spare 5th PC and logged in the missing 5th user from that PC. Then I was able to launch successfully, and afterwards I just retired that 5th player.
If you are starting a fresh game (i.e. not loading a previously saved game), the same error might be caused by the following: there are still open slots for human players, which haven't joined the game yet. Just switch them to "None" or to the Computer.
Regards, Lanzelot