Where are you getting this message? The game should be playable on Windows, as it was made in Game Maker Studio 2. However, it isn’t currently playable on Mac, because a Mac OS computer is unfortunately required to export for the platform even if you own a license to do so. (I’ve considered borrowing or purchasing a laptop to export later builds, so Mac users don’t despair.)
I have been using the itch.io app on desktop (in Windows 10). I prefer the desktop app over the browser site since installing games from the app keeps it all nice and tidy in a hidden folder instead of cluttering the downloads/desktop.
I wonder if it's an issue with certain file types--shortly after I made the original comment ,I was looking at a friend's game that also showed this message in the app, even though it was definitely a Windows-compatible executable. Now that I am looking at your game on the web browser, it lets me download it just fine.
This seems to be a stupid bug that I encountered with some other games as well. Kiana, that is the reason why I made my post here some months ago. The problem still exists, the itch app says "not available".
That is really strange! I wonder if it's been fixed in the latest version of GMS2. We converted the project for the original alpha build recently and it broke some stuff, but we fixed everything and re-exported it with some game feel improvements.