Grunt warning: "path must be a string" after, grunt sass command

I am new I listen something about Grunt, Sass, Bootstrap and Node.js, So I want to use it :P I visit https://designmodo.com/bootstrap-4-npm-grunt-sass-2/ and now i am trying to configure it, but now when I am write "grunt sass" in git, the message "Path must be a string.Received undefined" happen.

via J.Wołowski