Fix broken sudoers file – sudo: parse error in /etc/sudoers near line 21 [Ubuntu]

While providing sudo permission to a specific user did a mistake, saved and closed it before I realize. Later when tried to execute sudo -s encountered the below error message. I’m unable to open sudoers file and edit again to 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

How can I fix this error without booting into single-user mode?

Solution: Fix broken sudoers file

Modern GNU/Linux distribution systems come with a quite fast and easy way to fix the corrupted sudoers file and don’t require rebooting using a live CD, or physical access to the machine.

ubuntu@digi:$ pkexec vim /etc/sudoers
==== AUTHENTICATING FOR org.freedesktop.policykit.exec ===
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.