The latest version of the game is broken. The dev doesn't give a ♥♥♥♥ about it.
This guide shows you how to download the previous version of the game that actually worked

Open the Steam console by copy pasting this code into your browser's URL:
steam://nav/console
Press enter. A launch application should pop up. Select the Steam client and open the link.

Paste this command in the console:
download_depot 716110 716111 2601864388207252061
Press enter.

Wait for Steam to download the depot. You won't see any indication of progress, but you can tell it's downloading by looking at the network usage on your downloads page. The download can pause/resume if your connection goes out, but won't if you restart the client.

After the download is done, Steam will show you where the files were downloaded to.

Go to the original game's installation directory, and move the files somewhere else. Then go to where the depot files were downloaded to, and move everything over to the game folder.

Restart the game, Boom it ♥♥♥♥♥♥♥ works.

A big ♥♥♥♥ YOU to the dev.