Hi!
I just tried (using Firefox) and the download seems to work... maybe a temporary issue of the itch.io platform?
Here are the steps:
1) Click the "Download now" button in the project page: https://retrobits.itch.io/cave81
2) A popup should appear. Click the "No thanks, just take me to the downloads" link (although donations are welcome!)
3) A new page with a "Download" button should appear. Click the "Download" button and the "CAVE81_11.ZIP" file should now be downloaded. If this does not work, you can also click the "Downloads not starting?" link, enable the alternate download method and try again.
Please let me know if it works and thank you!