Could not calculate the upgrade An unresolvable problem occurred while calculating the upgrade. I've done upgrade 15.10 from 14.04. trying to upgrade 16 hit by this error

[Tom]

currently running 15.10 version of ubtuntu which was upgraded from 14.04, Moreover am new to ubuntu not a veteran. Help me to find out the root cause problem.
{
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 15.10
Package: ubuntu-release-upgrader-core 1:15.10.14.4
ProcVersionSignature: Ubuntu 4.2.0-38.45-generic 4.2.8-ckt10
Uname: Linux 4.2.0-38-generic x86_64
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Mon Jun 20 22:32:34 2016
InstallationDate: Installed on 2016-06-15 (5 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to wily on 2016-06-20 (0 days ago)
VarLogDistupgradeTermlog: