How to make scrollable pop up box?

https://codepen.io/anon/pen/dWaWor

When I click on the button "Creation Process" I cannot scroll in the lightbox.

The lightbox has a fixed position because when I used absolute the background messes up. The lightbox is a white background.

<section id="lightbox">
   <i id="x" class="fa fa-times-circle"aria-hidden="true"></i>
   <p class="large">hi</p>
</section>

>

#lightbox {
  height:100%;
  width:100%;
  display: none;
  position: fixed;
  top:0;
  left:0;
  background: #fff;
  z-index:1;
}

>

 var btn_process = document.getElementById('creation-process');
 var lightbox = document.getElementById('lightbox');
 var x = document.getElementById('x');

 btn_process.onclick = function () {
    lightbox.style.display = 'block';
};

x.onclick = function () {
    lightbox.style.display = 'none';
}