Most probably it is trying to reach body before DOM is ready. You should use DOMContentLoaded event for DOM manipulation.

https://developer.mozilla.org/en-US/docs/Web/API/Window/DOMContentLoaded_event

window.addEventListener('DOMContentLoaded', (event) => {
    document.body.appendChild(<element>);
});

Otherwise, it is better to share more code.

Whenever I try to use document.body.appendChild, it evaluates to null. Here is my code:

document.body.appendChild(c92w46);

I get this error:

main.js:96 Uncaught TypeError: Cannot read property 'appendChild' of null