我想用一个bootstrap导航栏做一个后台png，但它不会工作

<nav class="navbar navbar-expand-lg navbar-light bg-light">
  <a class="navbar-brand" href="#">Logo</a>
  <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>
  <<div class="collapse navbar-collapse" id="navbarNav">
    <ul class="navbar-nav">
      <li class="nav-item active">
        <a class="nav-link" href="#">Home</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">About</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Contact</a>
      </li>
    </ul>
  </div>
</nav>

.navbar {
  background-color: #f8f9fa;
}

.navbar-brand {
  font-size: 20px;
}

.navbar-nav .nav-item .nav-link {
  color: #000;
}

.navbar-nav .nav-item.active .nav-link {
  color: #fff;
  background-color: #007bff;
}