Why doesn't the script for a modal window?

Good day! There is a slider made for Slick Slider. Wanted to make pictures of the slider modal, meaning that when you click on the picture a window POPs up with a cross on it (then close it) and a picture, but for some reason does not work, I thought that thing in the script, I have tried to rewrite it with jQuery, but to no avail. Below point code.
HTML

<i class="fa fa-chevron-circle-left" aria-hidden="true"></i>
<i class="fa fa-chevron-circle-right" aria-hidden="true"></i>
<div id="myModal" class="modal">
 <span class="close">×</span>
 <img class="modal-content" id="img01">
</div>
<div class="gallery">
 <img class="img" src="images/interior_1.jpg" alt="#">
 <img class="img" src="images/interior_2.jpg" alt="#">
 <img class="img" src="images/interior_3.jpg" alt="#">
 <img class="img" src="images/interior_4.jpg" alt="#">
 <img class="img" src="images/interior_5.jpg" alt="#">
</div>

CSS

.modal {
 display: none; /* Hidden by default */
 position: fixed; /* Stay in place */
 z-index: 5; /* Sit on top */
 padding-top: 100px; /* Location of the box */
 left: 0;
 top: 0;
 width: 100%; /* Full width */
 height: 100%; /* Full height */
 overflow: auto; /* Enable scroll if needed */
 background-color: rgb(0,0,0); /* Fallback color */
 background-color: rgba(0,0,0,0.9); /* Black w/ opacity */
}

.modal-content {
 margin: auto;
 display: block;
 width: 80%;
 max-width: 700px;
 max-height: 500px;
}

.modal-content { 
 -webkit-animation-name: zoom;
 -webkit-animation-duration: 0.6 s;
 animation-name: zoom;
 animation-duration: 0.6 s;
}

@-webkit-keyframes zoom {
 from {-webkit-transform:scale(0)} 
 to {-webkit-transform:scale(1)}
}

@keyframes zoom {
 from {transform:scale(0)} 
 to {transform:scale(1)}
}

.close {
 position: absolute;
 top: 15px;
 right: 35px;
 color: #f1f1f1;
 font-size: 40px;
 font-weight: bold;
 transition: 0.3 s;
}

.close:hover,
.close:focus {
 color: #bbb;
 text-decoration: none;
 cursor: pointer;
}

Java Script

var modal = document.getElementById("myModal");

 // Get the image and insert it inside the modal
 var img = document.getElementsByClassName("img");
 var modalImg = document.getElementById("img01");
 img.onclick = function(){
 modal.style.display = "block";
 modalImg.src = this.src;
};

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

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

If who rummages in such things, please help, I have a couple of hours sat. but I can not understand why is not working.