Can't upgrade from LTS 18.04 to LTS 20.04

[Michael Ho]

Using the instructions at: https://ubuntu.com/blog/how-to-upgrade-from-ubuntu-18-04-lts-to-20-04-lts-today

> sudo do-release-upgrade -d –allow-third-party

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

This was likely caused by:
* Unofficial software packages not provided by Ubuntu
Please use the tool 'ppa-purge' from the ppa-purge
package to remove software from a Launchpad PPA and
try the upgrade again.

If none of this applies, then please report this bug using the
command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal. If
you want to investigate this yourself the log files in
'/var/log/dist-upgrade' will contain details about the upgrade.
Specifically, look at 'main.log' and 'apt.log'.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.37
ProcVersionSignature: Ubuntu 5.3.0-7648.41~1586790036~18.04~600aeb5~dev-generic 5.3.18
Uname: Linux 5.3.0-7648-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.14
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Thu May 7 11:08:52 2020
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-05-07 (0 days ago)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.