需要在导航栏内添加下拉列表

Question

我已经在该网站上工作了几天，试图为“位置”标签添加两个阶段的下拉列表，但无法使其正常工作。我将链接源到此评论。谁能找到我做错的地方，并在CSS上提供一些提示

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<header class="header" id="HOME">


    <nav class="navbar navbar-expand-md bg-dark navbar-dark fixed-top">
      <!-- Brand -->
      <a class="navbar-brand" href="#"><b><i>NS-MART</i></b></a>

      <!-- Toggler/collapsibe Button -->
      <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#collapsibleNavbar">
        <span class="navbar-toggler-icon"></span>
      </button>

      <!-- Navbar links -->
        <div class="collapse navbar-collapse" id="collapsibleNavbar">

            <ul class="navbar-nav">

              <li class="nav-item">
                <a class="nav-link" href="#HOME">HOME</a>
              </li>
               <li class="nav-item">
                <a class="nav-link" href="#">CATEGORIES</a>
              </li>
              <li class="nav-item">
                <a class="nav-link" href="#ABOUT US">ABOUT US</a>
              </li>


              <li class="nav-item">
                <a class="nav-link" href="#">LOCATIONS</a>
                  <ul>
                      <li><a href="#">item-1</a>
                           <ul><li><a href="#">subitem1</li></li><ul>    
                      </li>
                  </ul>
              </li>

              <li class="nav-item">
                <a class="nav-link" href="social.html">SOCIAL</a>
              </li>
              <li class="nav-item">
                <a class="nav-link" href="#TESTIMONIALS">TESTIMONIALS</a>
              </li> 
              <li class="nav-item">
                <a class="nav-link" href="#CONTACT-US">CONTACT-US</a>
              </li> 
              <li class="nav-item">
                <a class="nav-link" href="loginpage.html">SIGN IN</a>
              </li> 
              <li class="nav-item">
                <a class="nav-link" href="registerpage.html">REGISTER</a>
              </li> 
            </ul>
      </div> 
    </nav>