Add user in sudoers file

User name not found in Sudoers file.

Try this, it will definitely help you

1. Start terminal
2. apply following commands
     $ su -
     $ chmod u+rw /etc/sudoers

3. open
       sudoers file with  $ vi /etc/sudoers
                         or       $ gedit /etc/sudoers
Note : recommended is vi .

4. Add entry with user name i.e <user name> ALL = (ALL)ALL
    or what permission you want to give

5. chmod u-w /etc/sudoers

That's it .