You can play with 2 players (even solo), if the enter key doesn't do anything it looks like a bug, but I've never seen this problem and I'm surprised you have it on both Linux and Windows, I would like to help but I don't see a solution.
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. :)