update from 18.04 to 20.04 fail

[Yoichi Watanabe]

I wanted to upgrade my ubuntu 18.04 to 20.04 but failed.
I did:

$ sudo apt update
$ sudo apt upgrade
$ sudo apt dist-upgrade
$ sudo apt autoremove
$ sudo do-release-upgrade

and the error was
"An unresolvable problem occurred while calculating the upgrade."

There was a error in /var/log/dist-upgrade/main.log
2023-10-14 21:25:57,818 INFO installing linux metapackage: linux-generic
2023-10-14 21:25:57,818 DEBUG Installing 'linux-generic' (linux metapackage may have been accidentally uninstalled)
2023-10-14 21:25:57,829 DEBUG Installing 'ubuntu-minimal' (base meta package keep installed rule)
2023-10-14 21:25:57,870 DEBUG blacklist expr 'update-manager$' matches 'update-manager'
2023-10-14 21:25:57,870 DEBUG The package 'update-manager' is marked for removal but it's in the removal blacklist
2023-10-14 21:25:57,874 ERROR Dist-upgrade failed: 'パッケージ'update-manager'は「削除」と指定されていますが、削除ブラックリストに入っています。'
2023-10-14 21:25:57,891 DEBUG abort called

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.45
ProcVersionSignature: Ubuntu 5.4.0-150.167~18.04.1-generic 5.4.233
Uname: Linux 5.4.0-150-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.29
Architecture: amd64
CrashDB: ubuntu
Date: Sat Oct 14 21:28:20 2023
InstallationDate: Installed on 2020-04-24 (1268 days ago)
InstallationMedia: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190808)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=ja_JP.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
Symptom: ubuntu-release-upgrader
UpgradeStatus: Upgraded to bionic on 2023-10-14 (0 days ago)