An unresolvable problem occurred while calculating the upgrade. 18.04 to 20.04

[Ajith Thomas]

Cannot upgrade from 18.04 to 20.04 using the GUI upgrade method. It fails with "cannot calculate upgrade" and asks to "ppa-purge" to remove some packages.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.38
ProcVersionSignature: Ubuntu 4.15.0-112.113-generic 4.15.18
Uname: Linux 4.15.0-112-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.9-0ubuntu7.15
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Wed Jul 29 01:45:30 2020
InstallationDate: Installed on 2017-09-16 (1046 days ago)
InstallationMedia: Ubuntu 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-07-28 (0 days ago)
mtime.conffile..etc.update-manager.release-upgrades: 2020-07-29T01:03:38.114606