All the characters must be ready (with the button at the bottom) then a message appears saying that the game can be started, you must press "enter" or start on gamepad at this time
I can confirm that the Enter key does nothing. This is on both Linux and Windows and on the same machine (a new latop I bought last summer). I will try on my macBook tomorrow. There is no other issue with the Enter key anywhere on Linux and/or Windows in any app so this is as WTF as it gets, I guess. :)