Updating from 16.04 to 18.04 craches

[Isuru Weerasinghe]

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 1:16.04.30
ProcVersionSignature: Ubuntu 4.15.0-118.119~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-118-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.24
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Mon Oct 5 00:02:54 2020
InstallationDate: Installed on 2020-04-03 (183 days ago)
InstallationMedia: Ubuntu 16.04.6 LTS "Xenial Xerus" - Release amd64 (20190227)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2020-10-04 (0 days ago)
VarLogDistupgradeTermlog: