很难看到你的特定项目中发生了什么，因为我刚刚复制和粘贴了你的代码，它工作得很好，检查这个plunker：http://plnkr.co/edit/CFntB5mkshJ97x1aIGU1?p=preview
您可能需要验证是否正确导入（import {NgbModule} from '@ng-bootstrap/ng-bootstrap'）添加NgbModule模块：
@NgModule({ imports: [ BrowserModule, NgbModule ], declarations: [ App ], bootstrap: [ App ] }) export class AppModule {}个

最后，我把它作为Bootstrap 5。

<nav class="navbar navbar-expand-lg navbar-light bg-light">
  <a class="navbar-brand" href="#">Your App</a>

  <ul class="navbar-nav">
    <li class="nav-item">
      <a class="nav-link" href="#">Home</a>
    </li>

    <!-- Dropdown Menu -->
    <li ngbDropdown class="nav-item">
      <a
        class="nav-link"
        id="dropdownMenu"
        ngbDropdownToggle
      >
        Dropdown
      </a>
      <div ngbDropdownMenu aria-labelledby="dropdownMenu">
        <a ngbDropdownItem href="#">Item 1</a>
        <a ngbDropdownItem href="#">Item 2</a>
        <a ngbDropdownItem href="#">Item 3</a>
      </div>
    </li>
  </ul>
</nav>

字符串

如果你想你的导航栏切换，看看这个;

<nav class="navbar navbar-dark bg-inverse">
<button class="navbar-toggler hidden-sm-up" type="button" data-toggle="collapse" data-target="#navExample" aria-controls="navExample" aria-expanded="false" aria-label="Toggle navigation">
    &#9776;
</button>
<div class="collapse navbar-toggleable-xs" id="navExample">
    <ul class="nav navbar-nav">
        <li class="nav-item">
            <a href="#" class="nav-item nav-link">First link</a>
        </li>
        <li class="nav-item">
            <a href="#" class="nav-item nav-link">Second link</a>
        </li>
    </ul>
</div>

字符串
如果你只想在你的导航栏中有一个菜单，在官方网站的例子应该工作;

<div ngbDropdown class="d-inline-block">
    <button class="btn btn-outline-primary" id="dropdownMenu1" ngbDropdownToggle>Toggle dropdown</button>
    <div class="dropdown-menu" aria-labelledby="dropdownMenu1">
        <button class="dropdown-item">Action - 1</button>
        <button class="dropdown-item">Another Action</button>
        <button class="dropdown-item">Something else is here</button>
    </div>
</div>

型
确保你的html文件包含bootstrap（我使用bootstrap 4）。