Hello,
I'm sorry for the inconvenience, this shouldn't happen and the donation is not necessary to try the game.
I just tried and the download works so I don't really understand what happened in your case, but if it's still not working, here's a GDrive link where you'll find the demo files: https://drive.google.com/file/d/15onNP0OIqyuf710Smfyxmc5C41kBtBxe/view?usp=drive...
Once again, I'm sorry for your troubles. Anyway, I hope you'll enjoy the game!