Team is on it. Hopefully we can find a fix for ya. Can you get a screenshot of your error for us please?
Viewing post in "Cannot read property build of undefined"
To fix this, go to the "edit game" menu, then tick the Windows checkbox next to "Cattle-Castle-2-1.3.3.zip." (Doing the same for the MacOS version will fix the same issue for Mac users)
This error just means that the Itch Desktop App doesn't think any of the files are compatible with Nova25's operating system, and ticking that checkbox will fix it.
I was talking to the developer, sorry for the confusion, haha. If they want their game to work with the Itch.io desktop app, they need to tell it which files correspond to each operating system, and right now, both files are marked as not being compatible with any of them.
As for you, just install the game manually by going to Itch.io in your browser, clicking the arrow in the upper right, going to "my library," and downloading it there. Updates are infrequent enough that the desktop app doesn't really offer much benefit over a manual installation.