ubuntu 18.04 server gnome-session-flashback installation

sudo apt-get install gnome-core gnome-session-flashback vino

sudo apt-get install ubuntu-gnome-desktop gnome-session-flashback

sudo service lightdm start

Yes, you can install GNOME FlashBack session on 18.04 LTS:

sudo apt-get install gnome-panel

and then select GNOME FlashBack (Metacity) on login screen.

But as it is a server system I recommend to switch it back to console login as default target:

sudo systemctl set-default multi-user.target

and run GNOME FlashBack manually on demand:

sudo systemctl isolate graphical.target