Steam isn't downloading a game but it thinks it is ('missing executable' error message)

I contacted Steam support about the issue, and they said they were able to reproduce the issue, so it was not specific to my account or anything. They contacted the developer, and learned that the Mac OS version of the game is currently unavailable while the devs are working on updating the game to 64-bit (so that it will be compatible with Mac OS Catalina). The developers have since posted an update on the Steam store page describing the situation. They say the 64-bit version should be out ASAP, so the only thing we can do at this point is wait.

UPDATE: The game is now fixed and the 64-bit version is out. It can now be properly downloaded on Macs.