Bootstrap Navbar doesn't show the menu options after it collapses

I am using bootstrap 4. The navbar looks like this on a large screen: However, after it collapses on a small screen, it doesn't show the menu option when I click the menu button:

On a small screen, it should list the menu options once the button is clicked. How can I fix this issue? Here is my HTML and CSS code:

<!DOCTYPE html>
<html>
   <head>
       <title>Bootstrap Resize</title>
       <meta charset="utf-8"></meta>
       <meta name="viewport" content="width=device-width, initial-scale=1" ></meta>
       <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.5.2/css/bootstrap.min.css" />
       <script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.5.2/js/bootstrap.min.js"></script>
      
   </head>
   <style type="text/css">
    .navbar-custom {
     height: 54px; 
     background-color: #484848;
   }
   </style>

   <body>                 
    <nav class="navbar navbar-custom navbar-expand-lg navbar-dark fixed-top" id="mainNav">
      <div class="container">
        <a  style="font: 20px Arial,sans-serif" class="navbar-brand js-scroll-trigger" href="#">HOME</a>
        <button class="navbar-toggler navbar-toggler-right" type="button" data-toggle="collapse" data-target="#navbarResponsive" aria-controls="navbarResponsive" aria-expanded="false" aria-label="Toggle navigation">
          <span class="navbar-toggler-icon"></span>
        </button>

        <div class="collapse navbar-collapse" id="navbarResponsive">
          <ul style="font: 15px Roboto,sans-serif" class="navbar-nav ml-auto">
            <li class="nav-item">
              <a class="nav-link js-scroll-trigger" href="#about">LOREM</a>
            </li>
            <li class="nav-item">
              <a class="nav-link js-scroll-trigger" href="#expertise">LOREM</a>
            </li>
            <li class="nav-item">
              <a class="nav-link js-scroll-trigger" href="#portfolio">LOREM</a>
            </li>
            <li class="nav-item">
              <a class="nav-link js-scroll-trigger" href="#contact">LOREM</a>
            </li>
          </ul>
        </div>

      </div>
    </nav>
   </body>
</html>