Unable to upgrade from Ubuntu 16.04 to 18

[Ma Yiqiang]

Error info:
Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

 This was likely caused by:
 * Unofficial software packages not provided by Ubuntu
Please use the tool 'ppa-purge' from the ppa-purge
package to remove software from a Launchpad PPA and
try the upgrade again.

If none of this applies, then please report this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a 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'.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.32
ProcVersionSignature: Ubuntu 4.15.0-133.137~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-133-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.28
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Mon Feb 1 17:23:52 2021
InstallationDate: Installed on 2017-05-02 (1371 days ago)
InstallationMedia: Ubuntu 16.04.2 LTS "Xenial Xerus" - Release amd64 (20170215.2)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2021-02-01 (0 days ago)
VarLogDistupgradeTermlog: