An unknown error is causing the upgrade from 16.04 to 18.04 to fail. No specific error name displayed. It only points out that it is unresolvable

[Prabhat Telidevara]

Calculating the changes

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

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 16.04
Package: ubuntu-release-upgrader-core 1:16.04.32
ProcVersionSignature: Ubuntu 4.15.0-142.146~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-142-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.30
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Fri May 14 11:13:05 2021
InstallationDate: Installed on 2017-12-16 (1245 days ago)
InstallationMedia: Ubuntu 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2021-05-14 (0 days ago)