Apparently games that were made with Game Maker are having these problem. It's probably not on our end but we're still looking it up.
Try these fixes:
(Option 1)
Opting out of the Steam client beta:
go to Steam settings > Account > opt out of the beta.
(Option 2)
(This method won't count playtime and achievements, but it makes the game work)
Navigate to your Reigning Cats directory (right click on Reigning Cats in your Steam library > Properties > Local files tab > Browse local files).
You should see Reigning Cats.exe and a bunch of files.
Delete steam_api.dll and launch the game.