Bootstrap Hamburger菜单将无法在移动视图中打开

如何解决Bootstrap Hamburger菜单将无法在移动视图中打开

我已经工作了几天，试图弄清楚为什么无法在移动设备上打开菜单。我什至尝试完全删除我的并粘贴在bootstraps模板中-仍然不起作用。我在页面末尾，在标记之前，有jquery，popper.js和javascript插件。

<nav class="navbar navbar-expand-lg navbar-light bg-light">
        <a class="navbar-brand" href="index.html"><img class="logo" src="imgs/favicon.png" alt="my logo">
        <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav" aria-controls="#navbarNav" aria-expanded="false" aria-label="Toggle navigation">
            <span class="navbar-toggler-icon"></span>
        </button></a>
        <header class="collapse navbar-collapse" id="navbarNav">
            <ul class="navbar-nav">

                <li class="nav-item active">
                    <a class="nav-link" href="index.html">Home/About<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>
        </header>
    </nav>