Ah, it worked okay when I downloaded it a couple of weeks ago. It looks like it could be a temporary DropBox error, but if not then Mike Dailly might have removed it for some reason. I can't find any other sites with a download (and it's probably not a good idea to trust other sources anyway).
If you don't have a Spectrum Next to play the game on, then I'm happy for you to request a refund from itch.io support.