You ran into the same problem I did!
1. If you're on Itch, scroll to the top of the page
2. In the upper right is your profile, click the arrow next to it. Click "My Library"
3. On the left menu should be "My Purchases"
4. Find the game and copy the url of the download page and open it in a browser window. Outside of the itch app. It won't install Windows via itch app for some reason.