It says that TypeError: document.getElementById(...) is null

All these results in null:

document.getElementById('volume');
document.getElementById('bytesLoaded');
document.getElementById('startBytes');
document.getElementById('bytesTotal');

You need to do a null check in updateHTML like this:

function updateHTML(elmId, value) {
  var elem = document.getElementById(elmId);
  if(typeof elem !== 'undefined' && elem !== null) {
    elem.innerHTML = value;
  }
}

Make sure the script is placed in the bottom of the BODY element of the document you're trying to manipulate, not in the HEAD element or placed before any of the elements you want to "get".

It does not matter if you import the script or if it's inline, the important thing is the placing. You don't have to put the command inside a function either; while it's good practice you can just call it directly, it works just fine.

It means that the element with the id passed to getElementById() does not exist.