EDIT (Jan 11): I found "my" workaround (with apologies that it won't help others with the same problem).
The tar.gz distro of the software I want to run works fine, out of the box, on my old Ubuntu 16.04 system (which won't run flatpak either!). Since this is not software I expect to need very often, I'm going to run it there.
Many thanks to everyone who invested their time and energy in helping me with this.
I have a piece of software that I want to install that's distributed as a "flatpak". However, after installing the flatpak system in my Ubuntu Mate 20.04 system, it fails to load the package I want to use. I get this error:
$ flatpak install chirp-daily-20220103.flatpak
Note that the directories
'/var/lib/flatpak/exports/share'
'/home/simon/.local/share/flatpak/exports/share'
are not in the search path set by the XDG_DATA_DIRS environment variable, so
applications installed by Flatpak may not appear on your desktop until the
session is restarted.
** (flatpak install:41527): WARNING **: 20:31:39.113: Error unregistering authentication agent: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name :1.235 was not provided by any .service files
error: Authorization error: Message recipient disconnected from message bus without replying
Further, attempting to list flatpaks doesn't seem to work either, so I don't know what else I can try:
$ flatpak search vlc
No matches found
The above, as best I understand, should list options for installing vlc.
Thanks to all for the efforts to assist. Sadly as of today (Jan 10) there's no improvement. I followed N0rbert's guidance to install / upgrade from the PPA (my original install was directly from the core repos "sudo apt install flatpak"). The installation seemed to work, but on attempting the remote-add, it fails:
$ flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
** (flatpak remote-add:37255): WARNING **: 15:10:26.654: Error unregistering authentication agent: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name :1.139 was not provided by any .service files
error: Authorization error: Message recipient disconnected from message bus without replying
Is this possibly a Mate-specific thing? (I guess I'll try it on one of my other machines that has vanilla Ubuntu 20.04.)
Any suggestions?