how do run my js code in web without problem? [duplicate]

This question already has an answer here:

• Why does jQuery or a DOM method such as getElementById not find the element? 8 answers

So when I write my javascript code in VScode and try to run it in my page an error message appears in my console saying: "Uncaught TypeError: Cannot read property 'addEventListener' of null at app.js:2"

But when I write the same code in the console of the page directly the code is executed without any problems. The code is:

 var button = document.querySelector("button");
button.addEventListener("click",function(){
    alert("clicked!");});