make sure you have enough space and that you are downloading the correct file.
I do. 9 GB free should be enought. And I just press "install" button on the store's page.
update:
I updated the itch.io client and now I see the error "This title is hosted on an incompatible third-party website". and I no longer can press "install" button.
From there the page redirects me to mega.nz where the file is hosted, and asks me to download the "MEGA desktop app" to download the file. Because supposedly the firefox can't handle it.
Oooookaaaay...
Dev, can you please comment?