I'm sorry about the trouble. What system do you have/what version of the demo are you using? Have you tried downloading it again at a different time? Occasionally there's some issue that happens when downloading from Itch making the file not work, then when it's downloaded later it ends up being fine.
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Do you have an antivirus software? Sometimes that automatically prevents a new file from opening because it mistakenly thinks they're suspicious. Turning that off for just a couple minutes to launch the game might work out. And perhaps it didn't unzip properly. Maybe using a different unzipper than you did originally could help. I'd also recommend deleting everything, giving Itch even more time, and then trying to redownload again. Sometimes it takes a while to get past that.
Unfortunately, there aren't many other possibilities we might be able to help with. The demo has been openable for the general public when the download goes correctly. Sometimes there are other errors, but it should at least open. So if there are no issues downloading it from Itch, it was extracted from the zip without issues happening there, and it's the PC version of the demo for your Windows system, that'd mean the only part left to consider is the computer it's on. If the problem isn't the antivirus it's could be else, but we can only guess at what might be going on with the PC for it not start the file. At this point, asking in a Windows forum about what might prevent files from opening could be more helpful than we are. I'm sorry.