Comment 5 for bug 1887509

I've found lots of Ubuntu 20.04 users on forums, who meet this issue, for example:
https://discourse.ubuntu.com/t/is-ubuntu-software-going-to-be-remove-for-snap-snap-store/14542/123
https://discourse.ubuntu.com/t/is-ubuntu-software-going-to-be-remove-for-snap-snap-store/14542/141
There is also the same suggested solution like seb128 said - use files portal:
I tested the installation of .deb files again and I think I’m hitting the “sharing files to a snap via temp” issue: https://forum.snapcraft.io/t/sharing-files-via-tmp/1613/26 2
When I open the .deb files from the browser, I get the “Failed to install file: not supported” error but they work correctly when I download them to Home and then open them via Files.

Since this version of the snap store should only run on Ubuntu 20.04, maybe you can use files portal for this