Have you tried a different browser? Also, are you running Windows 64bit, Windows 32bit, Linux, or Mac?
I have been searching to solve your issue and I have found several things that worked for other people.
1. Try to download it when in incognito mode.
2. Turn off parental controls or other various security settings that prohibit downloads set by parents or administrators.
3. Try the app.
4. Keep trying to download it over and over. Some users had successful results after attempting to download several times.
5. Try it while signed out of your itch.io account.
6. I have mentioned this before, but try momentarily disabiling your antivirus. Malwarebytes and Bitdefender have been known to flag itch.io downloads.
7. Make sure you are on a stable internet connection. Itch.io does not support resumable downloads in the browser version.
I hope any of this helps. Also, are you able to download any other games off the website?