can't upgrade system due to some bug

[Cindy]

I'm trying to upgrade my 12.04 LTS to 14.04LTS and everything went fine till this part... I actually don't know if it's the update manager, but it says all this towards the end:

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

Calculating the changes

Calculating the changes

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.

Restoring original system state

Aborting
Reading package lists... Done
Building dependency tree
Reading state information... Done
Building data structures... Done

-------

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.19
Uname: Linux 3.3.8-24-generic i686
ApportVersion: 2.0.1-0ubuntu17.13
Architecture: i386
Date: Thu Jul 21 21:49:25 2016
GsettingsChanges:

MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 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 2016-07-22 (0 days ago)

[Brian Murray (brian-murray)]

The log files contain the following information:

2016-07-21 21:47:25,295 DEBUG none of the '['ubuntu-desktop', 'kubuntu-desktop', 'xubuntu-desktop', 'ubuntustudio-desktop', 'mythbuntu-desktop', 'kubuntu-netbook', 'lubuntu-desktop']' meta-pkgs installed
2016-07-21 21:47:25,296 DEBUG guessing 'ubuntu-desktop' as missing meta-pkg
2016-07-21 21:47:46,543 ERROR failed to mark 'ubuntu-desktop' for install (E:Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.)

Please install one of the meta-pkgs listed and then try upgrading again.

[Launchpad Janitor (janitor)]

[Expired for update-manager (Ubuntu) because there has been no activity for 60 days.]