/usr/bin/sudo must be owned by uid 0 and have the setuid bit set on Ubuntu 20.04
During the boot, press ‘ESC’ key to go to GRUB screen.
Choose first option “Ubuntu” and then press ‘e’ key to edit.
Append the string “systemd.unit=rescue.target” to the line which starts with ‘linux’ word.
pkexec chmod a=rx,u+ws /usr/bin/sudo