lubuntu 18.04.5 cannot partially upgrade

[fd ridhani]

i was running software upgrade on lubuntu 18.04.5, it was prompted to do partial upgrade which I was select OK.
After some times, its said "Can't install 'lubuntu-core': It was impossible to install a required package. Please report this as a bug using 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal".
Then it asked to close.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-release-upgrader-core 1:18.04.44
ProcVersionSignature: Ubuntu 5.4.0-73.82-generic 5.4.106
Uname: Linux 5.4.0-73-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.18
Architecture: amd64
CasperMD5CheckResult: skip
CrashDB: ubuntu
CurrentDesktop: LXDE
Date: Thu Jun 3 09:50:08 2021
InstallationDate: Installed on 2020-10-01 (244 days ago)
InstallationMedia: Lubuntu 18.04.5 LTS "Bionic Beaver" - Release amd64 (20200806.1)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: No upgrade log present (probably fresh install)
VarLogDistupgradeTermlog:

mtime.conffile..etc.update-manager.release-upgrades: 2021-06-03T09:16:02.654719

[Chris Guiver (guiverc)]

Thank you for taking the time to report this bug and helping to make Ubuntu better.

Lubuntu 18.04.5 was the last Lubuntu to use the LXDE desktop, and supported upgrades can only be performed via re-install, ie. release notes of 18.10 & later contain text like the following (found in https://lubuntu.me/focal-2-released/)

"Note, due to the extensive changes required for the shift in desktop environments, the Lubuntu team does not support upgrading from 18.04 or below to any greater release. Doing so will result in a broken system. If you are on 18.04 or below and would like to upgrade, please do a fresh install."

The package `lubuntu-core` doesn't exist in any release past-18.04 as it was specific to LXDE. ie.

$ rmadison lubuntu-core
 lubuntu-core | 0.38 | precise/universe | amd64, armel, armhf, i386, powerpc
 lubuntu-core | 0.55 | trusty/universe | amd64, armhf, i386, powerpc
 lubuntu-core | 0.65 | xenial/universe | amd64, armhf, i386, powerpc
 lubuntu-core | 0.65.3 | xenial-updates/universe | amd64, armhf, i386, powerpc
 lubuntu-core | 0.94 | bionic/universe | amd64, armhf, i386
 lubuntu-core | 0.94.1 | bionic-updates/universe | amd64, armhf, i386

Lubuntu 18.04 LTS was the end-of-the-road for the `lubuntu-core`(LXDE; https://packages.ubuntu.com/bionic-updates/lubuntu-core) package. It should have been removed, and your system made a Ubuntu system if you intended upgrade; as the Lubuntu system requires re-install as per documentation.