Upgrading from Ubuntu 12.04 to Ubuntu 14.04 does not work

[Muhammad Asif Arain]

An error occur while upgrading from Ubuntu 12.04 to 14.04. Here is the message:

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.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.19
ProcVersionSignature: Ubuntu 3.13.0-66.108~precise1-generic 3.13.11-ckt27
Uname: Linux 3.13.0-66-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.0.1-0ubuntu17.13
Architecture: amd64
Date: Fri Oct 30 21:23:37 2015
GsettingsChanges:
 com.ubuntu.update-manager check-new-release-ignore 'trusty'
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1446236488
 com.ubuntu.update-manager window-height 500
 com.ubuntu.update-manager window-width 600
InstallationMedia: Ubuntu 12.04.4 LTS "Precise Pangolin" - Release amd64 (20140204)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_US:en
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2015-10-30 (0 days ago)
VarLogDistupgradeTermlog:

[Launchpad Janitor (janitor)]

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