Error: Cannot set property 'onmousedown' of null - I am getting an Error if I want to set a property to an "getElementById" object

Question

The answer to this Question will probably be really easy and obvious. I want to asign some values on a Buttonclick via JavaScript. I get the following Error: "Uncaught TypeError: Cannot set property 'onmousedown' of null". I searched for maby 1 hour but didn't find anything.

The biggest problem is; THE EXACT SAME CODE WORKS ON A FIDDLE!!! https://jsfiddle.net/qnbdt2o9/

Am I missing something? I know, I could use JQuery, bit I want to do it in vanilla JS.

Does Someone see a Error in my Code?

var btn = document.getElementById("btn");

btn.onmousedown = function(){
  var startFitness = document.getElementById('startFitness').value;
  var maxFitness = document.getElementById('maxFitness').value;
  var maxSpeed = document.getElementById('maxSpeed').value;
  var minSpeed = document.getElementById('minSpeed').value;
  var agility = document.getElementById('agility').value;

  cosAtt = [startFitness, maxFitness, maxSpeed, minSpeed, agility];
  console.log(cosAtt);
}

<h4>Start Fitness <input type="text" id="startFitness"></h4>
<h4>Max Fitness <input type="text" id="maxFitness"></h4>
<h4>Max Speed <input type="text" id="maxSpeed"></h4>
<h4>Min Speed <input type="text" id="minSpeed"></h4>
<h4>Agility <input type="text" id="agility"></h4>

<button id="btn">+ Add</button>

EDIT:

I forgott to add a important Codeline to the question