On the error log that you posted, I can see on line 78 this:
operation not supported\ngithub.com/itchio/ox.Preallocate\n\t/root/go/pkg/mod/github.com/itchio/ox@v0.0.0-20200301160301-4e131878ba64/preallocate_others.go:28\ngithub.com/itchio/savior.(*FolderSink).Preallocate\n\t/root/go/pkg/mod/github.com/itchio/savior@v0.0.0-20200303195615-7cac7998294c/folder_sink.go:186\ngithub.com/itchio/savior/zipextractor.(*ZipExtractor).
This seems to be related to what you described, and seems to be an error because the project is in a .zip file and the app can’t extract it.
I tried on my machine, and I downloaded two projects, one with .tar.bz2 and one with .zip files, and they both work. Could it be that there is something misconfigured on your system? If I may ask, what distro are you using? Do you have a program installed to unzip files? (I’m not sure if the Itch app is using it’s own unzip-er or expects the system to provide one)