Distro 'ubuntu18.04' does not exist in our dictionary
Solution 1:
virt-install get's it's os information out of osinfo-db
If your os does not ship a recent version of osinfo-db you can manually download it from https://releases.pagure.org/libosinfo/ and import it. e.g.
wget https://releases.pagure.org/libosinfo/osinfo-db-20211013.tar.xz
osinfo-db-import -v osinfo-db-20211013.tar.xz
Sometimes even osinfo-db is not up to date yet, I found out I had to use fedora31 whilst fedora32 is out for a few weeks now, but there shouldn't be to much of a difference I believe.