This week launched a pretty major update for the game and some users on macOS have reported the game being unable to launch due to some internal changes to the game engine.
The good news is its an easy fix! Just right click and "uninstall" doomtrooper and then immediately re-install and Steam fixes everything nicely. Plus, all of your settings should remain intact! And of course, all your data/cards/decks are stored on our server so you never risk losing them.
Apologies for this inconvenience but it should be one of the last major changes to the underlying build we needed to do!
More Patch Notes coming soon regarding this update...