Update to LTS failed using do-release-upgrade

[Ian Murphy]

I tried both the command 'do-release-upgrade' and using the software upgrader GUI.
It informed me that there were 3rd party repositories that needed to be removed, so
using ppa-purge, I removed all that I could. Then, went into the software settings and
removed all the rest. I unchecked the boxes for everything except the main, universe, and even canonical's partner respositories but the upgrade to the newest LTS still failed

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.38
ProcVersionSignature: Ubuntu 5.4.0-48.52~18.04.1-generic 5.4.60
Uname: Linux 5.4.0-48-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: GNOME-Flashback:GNOME
Date: Wed Oct 7 17:38:37 2020
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-10-07 (0 days ago)
VarLogDistupgradeTermlog: