ReferenceError: document is not defined (JavaScript in VS Code)

Question

Im learning JS. Found a good video "JavaScript Programming - Full Course" and stucked at the 2nd part where I need to use DOM. I have the HTML document, in i refered to the JS file. Other stuff working well, but this:

let messageEL = document.getElementById("message-el")

Always get this error:

ReferenceError: document is not defined

I installed live server, and it works one time, than stops.

What can I do, to fix this? Do I need to download some extension? Please tell me the solution as simple as u can Im at the beginning of the learning path.

This is the code and the error