For people who have the same issue of querySelector or getElementById that returns the following error:
Uncaught TypeError: Cannot read property 'appendChild' of null
but you have a class name or id in the HTML…
If your script tag is in the head, the JavaScript is loaded before your HTML, therefore the element you are trying to access does not exist in the DOM yet. You will need to add defer to your script like so:
<script src="script.js" defer></script>