That just means it can’t be installed on the itch app because it wasn’t uploaded with the appropriate tools, some devs opt for this if they prefer other host sites.
To play you’ll have to open the site in a browser, use the download buttons to download the zip folder, extract it then run either the exe or the .sh file, exe if you run windows, .sh for linux. not sure about mac, same download but you’ll have to figure out which file to run on your own as i’ve never owned one.