To be able to help you, I'll need to know the exact error message.
That looks like an error with itch.io's client, so I'm not sure how useful I can be, it may be one for their tech support. But first, it might be worth trying just downloading it manually. The current build wasn't uploaded with their tools, so a couple of people have reported issues downloading it with the client. When I release Missy MZ, I'll put it up with their tools which should hopefully stop problems like this.