Are you downloading through the Itch app or the direct webpage? If using the app you can try the website, if website you could try using the app. Sometimes one method doesn't work for one reason or another, unfortunately.
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Oh if you're on Mac that's a problem with the way Apple manages apps. This is a third party demo build that can't be verified, and depending on your Mac the device might automatically refuse to let you have it. I'm really sorry. You can try to search for ways around that, but it'd be based on whichever specific Mac you have and I wouldn't be able to help.