Unable to upgrade from Ubuntu 16 to 18.

[Alastair Clarke]

I have 16.04.5 LTS. I have run sudo apt-get update, upgrade and autoremove (to remove broken packages).
When I run sudo do-release-upgrade, it starts off fine but then it aborts the upgrade. Here is the end of the output:

Checking package manager
Reading package lists... Done
Building dependency tree
Reading state information... Done

Calculating the changes

Calculating the changes

Could not determine 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

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.25
ProcVersionSignature: Ubuntu 4.4.0-135.161-generic 4.4.140
Uname: Linux 4.4.0-135-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Tue Sep 18 08:36:02 2018
InstallationDate: Installed on 2014-11-06 (1411 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2018-09-18 (0 days ago)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.