I have a dropdown in my navbar for my site and plainly, it is not dropping down. I have scoured my code and it honestly looks fine so I was wondering if someone here could point out the error that is causing this defect?

This is what happens when I try and click it (nothing): https://i.imgur.com/q8WHswi.gifv

And here is my complete navbar code:

HTML Code:
<nav class="navbar navbar-expand-lg navbar-dark">
    <a class="navbar-brand" href="#">
        <img src="icon.jpg" width="30" height="30" alt="mannaz"/>
    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
        <span class="navbar-toggler-icon"></span>
    <div class="collapse navbar-collapse" id="navbarSupportedContent">
        <ul class="navbar-nav mr-auto">
            <li class="nav-item">
                <a class="nav-link font-weight-bold" href="#">Home</a>
            <li class="nav-item dropdown">
                <a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
                <div class="dropdown-menu" aria-labelledby="navbarDropdown">
                    <a class="dropdown-item" href="#">Action</a>
                    <a class="dropdown-item" href="#">Another action</a>
                    <div class="dropdown-divider"></div>
                    <a class="dropdown-item" href="#">Something else here</a>