The app does not register that there is a downloadable file. Switching to a .zip file should help.

Can confirm, unfortunately the itch windows app can't handle RAR files, saying that it can't find an appropriate downloadable package to install the game. One has to go to the itch website instead and download. 

It might be due to the game page not having platform tags defined. Adding Windows (and any other applicable platforms) might fix the issue.