Gnome fails to lock the session
Issue
A user reports that when locking his desktop session with Ctrl + L or the Lock session button no lock screen appears at all.
Solution
One of the root causes for this can be overridden or inappropriate dconf settings. Optionally, it could also be a broken Gnome desktop installation.
Info
dconf is a low-level configuration system. Its main purpose is to provide a backend to gsettings which is also another command line tool to manage Gnome configuration settings for various components, e.g lock screen, display language, keyboard, networking, etc ...
In order to resolve this issue we will first start by upgrading and fixing any potentially broken packages on the system, so that Gnome gets automatically repaired if it had any missing dependencies or installation files.
sudo apt update && sudo apt upgrade