from 12.04 to 14.04 can't find files

[Stefania Sangiorgi]

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

what can I do?
 I use ubuntu 12-04 and i would like to upgrade to 14.04

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.18
ProcVersionSignature: Ubuntu 3.13.0-46.75~precise1-generic 3.13.11-ckt15
Uname: Linux 3.13.0-46-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.8
Architecture: amd64
Date: Wed Mar 11 10:41:14 2015
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1426066477
 com.ubuntu.update-manager window-height 590
 com.ubuntu.update-manager window-width 600
InstallationMedia: Ubuntu 12.04.2 LTS "Precise Pangolin" - Release amd64 (20130213)
MarkForUpload: True
PackageArchitecture: all
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2015-03-11 (0 days ago)
VarLogDistupgradeTermlog: