Sounds like the Itch app is broken.
The error that I get says that the download is from an incompatible third-party site. If you upload the latest free version to itch.io directly, it should start working.
The latest version is currently uploaded here. I have beta versions of the next update that will come soon.I'm not sure why it would say it's from an incomplete third party site. Perhaps the Itch app doesn't like mega links?
That is probably it. I wonder if itch.io has a list of "good" and "bad" third-party sites available for downloading through the itch client.
