Gnome lock screen does not work

Symptom: no lock screen under Gnome/Wayland. The rest of Gnome is functional.

Analysis: journalctl -b shows following lines:

gsd-media-keys[3703]: \
Couldn't lock screen: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: \
GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: \
The name org.gnome.Shell.ScreenShield was not provided by any .service files

Solution:

1. Check if gdm3 (Gnome Display manager) is installed
2. Run dpkg-reconfigure gdm3 and select it as a display manager

The issue is that only gdm3 can lock screen under Wayland. If you run lightdm or other display manager, they don’t provide required functions to the gnome-shell.