Fri Apr 2 21:54:53 UTC 2021

It looks like the mirror of the Ubuntu archive you are using is not
mirroring the release which you are trying to upgrade to. Please switch
to an official mirror of the Ubuntu archive and then try the upgrade
again.

>From VarLogDistupgradeMainLog.txt:

2021-03-29 15:10:51,087 DEBUG url_downloadable: http://ubuntu.mirror.datamossa.io/ubuntu//dists/focal/Release
2021-03-29 15:10:51,087 DEBUG s='http' n='ubuntu.mirror.datamossa.io' p='/ubuntu//dists/focal/Release' q='' f=''
2021-03-29 15:10:51,125 DEBUG error from httplib: 'HTTP Error 404: Not Found'

** Changed in: ubuntu-release-upgrader (Ubuntu)
       Status: New => Incomplete

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Foundations Bugs, which is subscribed to ubuntu-release-upgrader in
Ubuntu.
https://bugs.launchpad.net/bugs/1921673

Title:
  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'. Comment: I don't know what this means.

Status in ubuntu-release-upgrader package in Ubuntu:
  Incomplete

Bug description:
  I just thought it would upgrade from 18.04.5 LTS to 20 automatically
  but I get an error message and don't know what to do . What is PPA,
  PPA-Purge package and launchpad PPA? How do I know what is official
  and what is unoffical Ubuntu software?

  ProblemType: Bug
  DistroRelease: Ubuntu 18.04
  Package: ubuntu-release-upgrader-core 1:18.04.42
  ProcVersionSignature: Ubuntu 4.15.0-139.143-generic 4.15.18
  Uname: Linux 4.15.0-139-generic x86_64
  ApportVersion: 2.20.9-0ubuntu7.23
  Architecture: amd64
  CrashDB: ubuntu
  CurrentDesktop: ubuntu:GNOME
  Date: Mon Mar 29 15:13:23 2021
  InstallationDate: Installed on 2017-04-11 (1447 days ago)
  InstallationMedia: Ubuntu 16.04.2 LTS "Xenial Xerus" - Release amd64 (20170215.2)
  PackageArchitecture: all
  SourcePackage: ubuntu-release-upgrader
  UpgradeStatus: Upgraded to bionic on 2021-03-29 (0 days ago)
  VarLogDistupgradeTermlog:

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