My event listener won't work

Question

I keep getting an error whenever i write the addEventListener. Considers it null. What am i missing?
https://code.sololearn.com/WSh1xoCXgela/?ref=app

Answer 1

https://www.sololearn.com/post/90825/?ref=app
https://www.sololearn.com/discuss/2452002/?ref=app

Answer 2

Try

document.onload = () => {
//Js code here
}