which file are you trying to download? There are three total. PC, MAC, and MARKET.
Any of the above. I'm using the Itch.io app on my desktop and either via the 'install' button down on the lower right or by clicking on the 'download' button on the page and then going to the selections and selecting any of them, the app still just has a blank thing.
I'd imagine if I went the the page outside the app, it'd be available but for some reason you're not supported on the actual app/manager itself. Even tried checking the 'enable alternative download' box.