Accidentally I had ran a command sudo chmod -R 777 / on my Ubuntu/LINUX PC and it was just starting of problems! 

I can't run command with sudo, can't add/remove any admin or user. Because the current root user has no permission to authenticate. 

When I run any command starting with sudo, the following errors found:  
sudo: /etc/sudoers is world writable
sudo: no valid sudoers sources found, quitting
sudo: unable to initialize policy plugin
When I try to authenticate, it shows: 

Incorrect permission on /usr/lib/policykit-1/polkit-agent-helper-1(need to setuid root)

How I resolved this error:
  • Restart your PC
  • Go to Recovery Mode
  • Run the following command: 
    chmod -R 755 /
  • Reboot
  • Try again any command starting with sudo. Now you will find only error: /usr/bin/sudo must be owned by uid 0 and have the setuid bit set
  • To solve this again restart your PC
  • Go to Recovery Mode
  • Run the following command: 
    chown root:root /usr/bin/sudo && chmod 4755 /usr/bin/sudo
  • Reboot
Boom! It solved.