An unresolvable problem occurred while calculating the upgrade.

[christina]

not able to upgrade from 12.04 to 14.04.the error message is 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

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.22
ProcVersionSignature: Ubuntu 3.13.0-117.164~precise1-generic 3.13.11-ckt39
Uname: Linux 3.13.0-117-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.15
Architecture: amd64
Date: Sat Jun 15 04:21:26 2019
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1560551870
 com.ubuntu.update-manager show-details true
 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_IN:en
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_IN
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2019-06-14 (0 days ago)