'Failed to find session configuration default' (lightdm.log)

You probably did not set the user-session in LightDM's config file, e.g.

# /etc/lightdm/lightdm.conf
[SeatDefaults]
user-session=plasma-kde
...

therefore LightDM looks into the user's ~/.dmrc file and finds

Session=default

and hence looks for /usr/share/xsessions/default.desktop which is missing.

sure, thread is pretty old - but anyway: stumbled over this thread, when trying to fix lightdm not starting on a bananapi M3 (armbian) - solution was to apt install lightdm-gtk-greeter (which was not pulled in when installing lightdm) maybe this helps someone :)