It's an issue I've encountered with the Desktop Itch app. If the build cannot be identified as Windows/Linux, Mac, or Android (signified by the little Windows/Linux, etc. logo you usually see next to the .zip file names on the game page), then the Itch app won't let you download the build because it can't identify the operating system that the files belong to. Usually this gets fixed with time or if the developer assigns the OS type. I don't for sure which one fixes the issue, but please remain patient.