You probably have a slightly older version of Python installed. f-strings (where I use the curly brackets) were added in Python 3.6. I'll add the version requirement to the installation guide.
I've had some issues with the opponent's turn not ending a few times (I think they get stuck in an infinite loop not knowing what to do or something). Not specifically with unresponsiveness - do you mean that the clicks just take a few times to register? I'll look into it if/when I get around to making an updated version.
And yeah, it's a bit unbalanced sadly. I wanted to add spell/action/whatever cards which would help, but didn't have time. So long as the game only has the crewmate cards, one of them will always have more attack than everything else's defence (otherwise there'd be an invincible card, which there was for a little while during testing, lol). Maybe there should have just been less of that card in the deck. Thanks for the feedback!