An unresolvable problem occurred while calculating the upgrade.

[Manaf PM]

I was trying to upgrade ubuntu from 18.04 to 20. I am getting the below error during upgrade

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

This was caused by:
* Upgrading to a pre-release version of Ubuntu
This is most likely a transient problem,
please try again later.
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'.

Restoring original system state

Aborting
Reading package lists... Done
Building dependency tree
Reading state information... Done

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: 2.20.9-0ubuntu7.17
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: ubuntu:GNOME
Date: Mon Oct 5 14:18:21 2020
InstallationDate: Installed on 2020-03-17 (201 days ago)
InstallationMedia: Ubuntu 18.04.4 LTS "Bionic Beaver" - Release amd64 (20200203.1)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2020-10-05 (0 days ago)