Comment 3 for bug 1861631

This error message is shown not to alert users, but rather make users report these issues to the distributor, as this particular issue means the distributor has a mistake in their metadata somewhere.
In this case it's not Ubuntu's fault directly, but rather GNOME Software injecting invalid metadata.
This is fixed upstream with commit https://gitlab.gnome.org/GNOME/gnome-software/commit/7c1620990b6361fac370b76c9828f8f77ff77a72 which should land in the next Ubuntu release naturally (but may be cherry-picked to make this issue go away even sooner).