I've been working on building a responsive site using Bootstrap, and the navbar is giving me issues. Right now I have it set to toggle when the screen size shrinks, and while it does create a button for a dropdown menu, the menu doesn't actually drop down when the button is clicked. I've tried looking through other answers to similar problems and I'm not seeing anything specifically helpful to this issue. I'm using Bootstrap 4 (https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/css/bootstrap.min.css)
And here is the code for my navbar:
<nav class="navbar navbar-expand-sm navbar-dark bg-dark">
<a class="navbar-brand" href="index.html">First Last</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarToggleExternalContent"
aria-controls="navbarToggleExternalContent" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarSupportedContent">
<ul class="navbar-nav mr-auto">
<li class="nav-item active">
<a class="nav-link" href="./index.html">Home <span class="sr-only">(current)</span></a>
</li>
<li class="nav-item">
<a class="nav-link" href="./portfolio.html">Portfolio</a>
</li>
<li class="nav-item">
<a class="nav-link" href="./contact.html">Contact</a>
</li>
</ul>
</div>
</nav>
I'm sorry if this is a stupid question, I'm relatively new to coding.