The code in the video contains a lot more than you have here. Your code contains the prototype* for stepperRotate() in the line just above setup() but you don't have the function itself.
There's a link in that youtube that takes you to the code where you will see the function just below loop().
*at least I think it's called a prototype, I never use them, since the IDE doesn't insist on them and afaik takes care of the lack thereof under the hood