upgrade from ubuntu 16.04 to ubuntu18.04 failed

[marinalan]

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 16.04
Package: ubuntu-release-upgrader-core 1:16.04.25
ProcVersionSignature: Ubuntu 4.15.0-36.39~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-36-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Mon Oct 15 13:29:44 2018
InstallationDate: Installed on 2017-11-20 (329 days ago)
InstallationMedia: Ubuntu 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2018-10-15 (0 days ago)