I'm not able to upgrade from 16.04 LTS to 18.04 LTS

[Luca Bascetta]

I tried twice doing the release upgrade from one LTS to another one but all the times it failed with the following error:

Calculating the changes

Calculating the changes

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

This can be caused by:
* Upgrading to a pre-release version of Ubuntu
* Running the current pre-release version of Ubuntu
* Unofficial software packages not provided by Ubuntu

If none of this applies, then please report this bug using the
command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.

Restoring original system state

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

I see other users had a similar issue.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.26
ProcVersionSignature: Ubuntu 4.4.0-141.167-generic 4.4.162
Uname: Linux 4.4.0-141-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CrashDB: ubuntu
Date: Tue Jan 8 15:55:34 2019
InstallationDate: Installed on 2016-05-19 (964 days ago)
InstallationMedia: Ubuntu-GNOME 16.04 LTS "Xenial Xerus" - Release amd64 (20160421)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2019-01-08 (0 days ago)