I'm unable to upgrade to Ubuntu 16.04 from 14.04.

[Victor Adio]

After trying to upgrade from the terminal, it reports:

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.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.9
ProcVersionSignature: Ubuntu 3.13.0-117.164-generic 3.13.11-ckt39
Uname: Linux 3.13.0-117-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.23
Architecture: amd64
CrashDB: ubuntu
Date: Mon Apr 10 17:50:57 2017
InstallationDate: Installed on 2014-08-09 (974 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_NG:en
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_NG
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
Symptom: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2017-04-10 (0 days ago)