I had a very similar problem (with bionic). I was able to upgrade to FreeIPA 4.3.1 by installing version 3.36.1 of the package libnss3, from the cosmic repositories. There is a bug in the version 3.35 distributed with bionic (see free online bettinghttps://bugzilla.redhat.com/show_bug.cgi?id=1568271) which prevents upgrades (and not only... installing a replica IPA server was also failing for me).
** Bug watch added: Red Hat Bugzilla #1568271 https://bugzilla.redhat.com/show_bug.cgi?id=1568271 -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1800631 Title: ipa-server-upgrade fail To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/freeipa/+bug/1800631/+subscriptions -- ubuntu-bugs mailing list firstname.lastname@example.org https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs