How to fix broken sudoers file – sudo: parse error in /etc/sudoers in Ubuntu?

Scenario:

While trying to execute sudo -s have you encountered the below error message. Also are you unable to open sudoers file to edit and fix it.

ubuntu@digi:$ sudo -s
>>> /etc/sudoers: syntax error near line 21 <<<
sudo: parse error in /etc/sudoers near line 21
sudo: no valid sudoers sources found, quitting
sudo: unable to initialize policy plugin

Let's fix this error without booting into single-user mode. 

To fix broken sudoers file or corrupted sudoers file and don’t require rebooting using a live CD, or physical access to the machine. 

Solution: Fix broken sudoers file 

ubuntu@rootadminz:$ pkexec vim /etc/sudoers

Authentication is needed to run `/usr/bin/vim' as the super user
Authenticating as: Ubuntu (ubuntu)
Password:

It would prompt for the password. Authenticate yourself with the password for the user you are trying to authenticate and then it will open /etc/sudoersfile in your favourite editor (in my case vim) as root, and you can fix your /etc/sudoers file.

Done!