What is wrong with this JavaScript code please!!

Question

Im trying to add an event listener to a button in order to carry out a certain function but it has an error. It says that it cannot read properties of null,i dont understand.
var btn=document.getElementById("button");
btn.addEventListener('click',add);
Function add(){
alert("worked");
}

Answer 1

This post might help you understand why
https://www.sololearn.com/post/90825/?ref=app

Answer 2

Please link your complete code.
We can only add the event, once the html element is found – we need to "wait" for the DOM to be loaded.
There is a typo btw: "function", not "Function"

Answer 3

Michael Kamau please share complete code

Answer 4

Thank you Ipang,the post you shared has helped me understand something