Can't upgrade to 18.04 from 16.04

[Bala]

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 1:16.04.12
ProcVersionSignature: hostname 4.4.0-135.161-generic 4.4.140
Uname: Linux 4.4.0-135-generic x86_64
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Mon Sep 10 19:02:50 2018
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2018-09-10 (0 days ago)
mtime.conffile..etc.update-manager.release-upgrades: 2018-09-10T17:31:46.460369