สอบถามเกี่ยวกับ modal form หน่อยครับ อยากให้ดู code บางส่วนให้หน่อยว่าตรงไหนผิดครับ

<nav id="nav">
				<ul class="links">
					<li><a href="index.html">Home</a></li>
					<li><a href="#" data-toggle="modal" data-target="#register">Register</a></li>
					<li><a href="elements.html">Elements</a></li>
				</ul>
			</nav>

			<div class="modal fade" id="register" tabindex="-1" role="dialog" aria-labelledby="register" aria-hidden="true">
					<div class="modal-dialog modal-dialog-centered" role="document">
					  <div class="modal-content">
						<div class="modal-header">
						  <h2 class="modal-title" id="register">Register</h2>
						</div>
						<div class="modal-body">
								<form action="/action_page.php">
									<div class="form-group">
									  <label for="username">Username:</label>
									  <input type="text" class="form-control" id="username">
									</div>
									<div class="form-group">
									  <label for="pwd">Password:</label>
									  <input type="password" class="form-control" id="pwd">
									</div>
									<div class="form-group">
									  <label for="email">Email:</label>
									  <input type="email" class="form-control" id="email">
									</div>
									<div class="form-group">
									  <label for="firstname">Firstname:</label>
									  <input type="text" class="form-control" id="firstname">
									</div>
									<div class="form-group">
									  <label for="lastname">Lastname:</label>
									  <input type="text" class="form-control" id="lastname">
									</div>
									<br>
									<button type="submit" class="btn btn-primary">Submit</button>
								  </form>
						</div>
					  </div>
				</div>
			</div>