[Bug 1909743] [NEW] updating error from ubuntu 16.04 to 18.04

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when I update ubuntu 16.04 to ubuntu 18.04 on linux4.4, I found the following error. So I would like to request any advice to solve this error.
*Calculating the changes

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 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'. 


Restoring original system state

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.32
ProcVersionSignature: Ubuntu 4.4.0-197.229-generic 4.4.241
Uname: Linux 4.4.0-197-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.28
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Thu Dec 31 18:30:34 2020
InstallationDate: Installed on 2015-06-01 (2039 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Build amd64 LIVE Binary 20140422-09:40
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to xenial on 2020-12-31 (0 days ago)

** Affects: ubuntu-release-upgrader (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug dist-upgrade xenial
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updating error from ubuntu 16.04 to 18.04
https://bugs.launchpad.net/bugs/1909743
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