Bootstrap 4 Modal code causing problems with website?

Question

Hello I'm new to coding and added modals to a website. In preview the website works great, but when it's online it doesn't work. Is this caused by the code I did for the modals? Or could this be the export settings maybe?

All I changed to the website is adding modals and changed some images. Please answer as simple as you can. I barely know anything about building websites.

Thanks so much in advance.

<!-- Button to Open the Modal -->
<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#myModal19">
   Toelichting
</button>

<!-- The Modal -->
<div class="modal" id="myModal19">
  <div class="modal-dialog modal-dialog-scrollable">
    <div class="modal-content">

      <!-- Modal Header -->
      <div class="modal-header">
        <h1 class="modal-title">Toelichting</h1>
        <button type="button" class="close" data-dismiss="modal">×</button>
      </div>

      <!-- Modal body -->
      <div class="modal-body">
        <p>text</p>
        <p>text</p>
        <p>text</p>
        <p>text</p>
      </div>

      <!-- Modal footer -->
      <div class="modal-footer">
        <button type="button" class="btn btn-danger" data-dismiss="modal">Close</button>
      </div>
    </div>
  </div>
</div>