problems with collapse, navigation is impossible to position in the center, the button icon is also not displayed, although everything for it is connected and spelled out

<div class="container"> <div class="navbar-header"> <button class="navbar-toggle" type="button" data-toggle="collapse" data-target="#bs-navbar"> <span class="sr-only">Toggle navigation</span> <span class="icon-bar"></span> <span class="icon-bar"></span> <span class="icon-bar"></span> </button> </div> <div class="collapse navbar-collapse" id="bs-navbar"> <ul class="nav navbar-nav"> <li> <a href="#">о процедуре</a> </li> <li> <a href="#">отзывы</a> </li> <li> <a href="#">центр обучения</a> </li> <li> <a href="#">контакты</a> </li> </ul> </div> </div> <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"></script> <script src="assets/js/bootstrap.js"></script>