I was not able to download from the itch.io app on Ubuntu too. I've got errors similar to yours. I'm still researching about it. It seems to be a matter with butler, the software itch.io offers for developers to build the distribution files, but it's not mandatory to use it, so it surprises me that this is happening.
On Windows, the app downloads fine.
Were you able to download the full game from the website and run it on Ubuntu?
From the website it worked fine. Could just login, download the game, set permission as executable, double click to launch.
So the actual file runs fine.
The only thing I am wondering is whether butler is trying to handle the binary as an installer, while the game does not appear to need any installing, but can be run directly. So installing from the app fails, since the file is not an installer and butler tries to handle it like one.
Thanks, I'm relieved to know you are able to download the game.
I've juste opened a topic in the forums. Unfortunately, there are other topics with similar problems, and it doesn't seem to be solved...
https://itch.io/t/1231627/download-errored-no-manager-for-installer-unknown
