https://www.steemimg.com/images/2016/08/10/security-146536_64032e98.png

Never edit your /etc/sudoers using a standard editor like 'vi' or 'nano', always use a special editor like 'visudo'. 

If you enter a typo or parse error into this file using a standard editor, in many cases you won't be able to re-edit this file (because you need "sudo" privileges to edit this file, and sudo relies on a valid /etc/sudoers file ), the safe way to edit this file is to use 'visudo',  it will parse the errors before saving this file...