JSFiddle - React, Tailwind, and code Playground

HTML

<div class="columns">
<ul class="price">
<li class="header">Hard Drive Format</li>
<li class="grey">£10</li>
<li>Format Hard Drive</li>
<li>Removes ALL Files From Drive</li>
<li>Fresh Install Windows</li>
<li class="grey"><button 
onclick="document.getElementById('myModal').style.display='block'">Sign 
Up</button></li>
</ul>
</div>

<!-- The Modal -->
<div id="myModal" style="display:none;" class="modal">

<!-- Modal content -->
<div class="modal-content">
<span class="close">&times;</span>
<p>Some text in the Modal..</p>
</div>

</div>

JavaScript

// Get the modal
var modal = document.getElementById('myModal');

// Get the button that opens the modal
var btn = document.getElementById("myBtn");

// Get the <span> element that closes the modal
var span = document.getElementsByClassName("close")[0];

// When the user clicks the button, open the modal 
btn.onclick = function() {
modal.style.display = "block";
}

// When the user clicks on <span> (x), close the modal
span.onclick = function() {
modal.style.display = "none";
}

// When the user clicks anywhere outside of the modal, close it
window.onclick = function(event) {
if (event.target == modal) {
    modal.style.display = "none";
}
}