Step 1: Login into your account and open terminal
Step 2: Type the following command to setup password for root account
sudo passwd root
Step 3: Open file /etc/pam.d/gdm-password
sudo nano /etc/pam.d/gdm-password
Step 4: Comment out the following line by placing # infront of it
#auth required pam_succeed_if.so user != root quiet_success
Step 5: sudo gedit /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf
Step 6: At the end of the file append greeter-show-manual–login = true
Step 7: Reboot the server
Enable root user in Ubuntu 20.04