can't upgrade to 18.04 from 16.04

[Cheolmin Jeon]

I tried to upgrade the version of ubuntu to 18.04 from 16.04.
But I failed with this message

'Could not calculate the upgrade'
'An unresolvable problem occurred while calculating the upgrade.'
'If none of this applies, then please report this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in terminal. If you want to investigate this yourself the log files in '/var/log/dist-upgrade/ will contain details about the upgrade. Specifically, look at 'main.log' and 'apt.log'.'

I am beginner of ubuntu.

plz help me.

lsb_release -rd
---------------------------------------
Description: Ubuntu 16.04.6 LTS
Release: 16.04
---------------------------------------

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.27
ProcVersionSignature: Ubuntu 4.4.0-166.195-generic 4.4.194
Uname: Linux 4.4.0-166-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.20.1-0ubuntu2.21
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Mon Nov 11 11:55:22 2019
InstallationDate: Installed on 2017-09-21 (781 days ago)
InstallationMedia: Ubuntu-Server 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2019-11-11 (0 days ago)
VarLogDistupgradeTermlog: