This is the common issue faced by Linux users. The issue is as, while running command with help of sudo raises following exceptions
sudo: /etc/sudoers is mode 0442, should be 0440
sudo: no valid sudoers sources found, quitting
sudo: unable to initialize policy plugin
So presenting the solution for the same, run the following command on your terminal.
pkexec chmod 0440 /etc/sudoers
Hope it will help you :)