I explained the bug here, https://trisquel.info/en/issues/12671
You can manually add Unity to the greeter by running the attached file using sudo.
echo '[GNOME Session] Name=Ubuntu RequiredComponents=unity-settings-daemon;compiz; DesktopName=Unity' > /usr/share/gnome-session/sessions/ubuntu.session echo '[SeatDefaults] user-session=ubuntu' > /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf echo '[Desktop Entry] Name=Ubuntu Comment=This session logs you into Ubuntu Exec=gnome-session --session=ubuntu TryExec=unity Icon= Type=Application X-LightDM-DesktopName=Unity X-Ubuntu-Gettext-Domain=gnome-session-3.0' > /usr/share/xsessions/ubuntu.desktop
