Comment 2 for bug 1930648

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.