Is it possible to install Lubuntu with the Ubiquity installer used by Ubuntu?
I can't setup full disk encryption correctly with the Calamares installer of Lubuntu 19.04; it has no option to use the entire disk and setup LVM encryption.
Instead of trying to make the Ubiquity installer work with Lubuntu, I'd suggest manually encrypting the entire drive after completing the normal installation of Lubuntu. The Ubuntu Community forum has a detailed how-to article with support for LUKS, LVM, encrypted Boot, and manual partitioning, among other features.
This feature request indicates the Calmares installer used by Lubuntu does not yet have full disk encryption.