can't update from 16.04 LTS to 18.04 LTS

[chj]

I'm wondering if this is caused by some software downloaded not from Ubuntu official.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.26
ProcVersionSignature: Ubuntu 4.15.0-43.46~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-43-generic x86_64
NonfreeKernelModules: wl nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Sun Jan 20 11:00:53 2019
InstallationDate: Installed on 2018-12-27 (23 days ago)
InstallationMedia: Ubuntu 16.04.5 LTS "Xenial Xerus" - Release amd64 (20180731)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2019-01-20 (0 days ago)
VarLogDistupgradeTermlog:

mtime.conffile..etc.update-manager.release-upgrades: 2019-01-20T10:45:12.465805