I have a new notebook with Ubuntu 20.04.1 where I migrated my KiCAD project files and libraries to. Since they are work in progress, I would like to keep KiCAD 5.1.12, but now only 6.0.2 is offered at the KiCAD website. How can I install KiCAD 5.1.12?
I assume you removed the 6.0 packages, but did you also remove the 6.0 repository specification? The package manager will always try to install the highest version available, so if the 6.0 repository is still enabled you will get 6.0 again.