Root user login in gnome is disabled on Debian system by default for security reasons. If you want to enable it for specific reasons then follow the below steps : 1. Login into Debian system 2. launch the Root Terminal from Applications -> Accessories. 3. Authenticate with password to launch the Root Terminal. 4. Open "/etc/pam.d/gdm3" with vi. #vi /etc/pam.d/gdm3 5. Look for "auth required pam_succeed_if.so user != root quiet_success" , comment it out as below. #auth required pam_succeed_if.so user != root quiet_success 6. Save the modification with Esc+w+q+Enter keystroke. 7. Restart or reboot Debian system. 8. Login with root user in gnome. You should be now able to login successfully.