Upgrading to 20.04, but getting "Please install all available updates for your release before upgrading."

Solution 1:

Between 18.04 and 20.04 Lubuntu has switched desktop environments. Lubuntu used to use LXDE but has switched to LXQt in recent releases.

Switching a DE is a huge change. It involves changing out dozens of core packages that all must work together in harmony to give you a working desktop environment. It's hard to account for all of the possible ways that the new software will work with all of your existing configurations for the LXDE desktop environment.

For this reason, upgrading from Lubuntu 18.04 to Lubuntu 20.04 requires a clean installation from external installation media. The typical upgrade path is unsupported.

The release notes for Lubuntu 20.04 state:

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.

You can download the new version of Lubuntu at lubuntu.me.