A quick workaround: start the tutorial and exit out to the main menu - you don't actually have to complete it for `Play` to unlock.
I made it mandatory because the last time I made a card game for Ludum Dare, *everybody* was confused about the rules. I might change it to "Are you *sure* you want to skip the tutorial?" prompt at some point.
edit: I just made some changes to address that. New game is no longer disabled, but if you click it, it'll recommend you do the tutorial - you can opt to click 'cancel' from there and go straight to a regular game. Hopefully this achieves both goals!