I am trying to make a Drupal 8 theme from an existing HTML page. The page uses the Bootstrap 4 menu.

I was able to get the normal version (desktop) of the menu working by copying menu--main.html.twig from the Drupal 8 Bootstrap Barrio theme. I added the following code to the file:

<nav class="navbar navbar-expand-lg navbar-light">

<a class="navbar-brand" href="/"><img alt="" src="/themes/themname/images/logo.png" style="max-width:150px;height:auto;" /></a><button aria-controls="navbarSupportedContent1" aria-expanded="false" aria-label="Toggle navigation" class="navbar-toggler" data-target="#navbarSupportedContent1" data-toggle="collapse" type="button"></button>



And when resizing the page the menu is disapearing as intentended.

I am having trouble to get the mobile menu working. The problem is that the button (hamburger - three lines) to open the mobile menu is not showing up.

The theme is loading the following .js files:

<script src="/core/assets/vendor/jquery/jquery.min.js?v=3.2.1"></script>
<script src="..path../js/popper.min.js?v=1"></script>
<script src="..path../js/bootstrap.min.js?v=1"></script>

As far as I understand there are no errors in browser console concerning this issue.

What might I still be missing?