can't upgrade from 12.04 latest to 14.04.5

[Kent McNaughton]

The upgrade from 12.04 Precise to 14.04.05 fails with a "Could not calculate the upgrade" error. Though I don't believe I'm running "Unofficial software packages not provided by Ubuntu," I can't be certain of this, as I've been running 12.04 now for five years on this machine. How to tell if this could be the problem? After using the GUI "Update Manager", I've run "sudo apt-get update", "sudo apt-get upgrade", and "sudo apt-get dist-upgrade". The response is "Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded."

I'm running Ubuntu 3.2.0-125-generic #168-Ubuntu SMP x86_64 on a several year old Intel CPU.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.21
ProcVersionSignature: Ubuntu 3.2.0-125.168-generic 3.2.79
Uname: Linux 3.2.0-125-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.15
Architecture: amd64
Date: Sat Apr 1 15:46:00 2017
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
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 2017-04-01 (0 days ago)
VarLogDistupgradeTermlog: