привет место Tallk мне, где Erorr в этом коде

Question

$(document).ready(function(){  
    $('#nav-b  .navbar-item li a').click(function(){    
        $('#nav-b .navbar-item li a').removeClass('active');            
        $(this).addClass('active');  
    }); 
});

<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js"></script>
<nav class="navbar navbar-expand-lg navbar-light bg-light  dark bg-dark ">
	<div class="container">
		<div class="navbar-brand"> <h5>Mohammed <span>Hassan</span></h5></div>
		<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
			<span class="navbar-toggler-icon"></span>
		</button>


		<div class="collapse navbar-collapse" id="navbarSupportedContent">
			<ul class="navbar-nav ml-auto" id="nav-b">
				<li class="nav-item active mr-auto">
					<a class="nav-link" href="#"   >Home <span class="sr-only">(current)</span></a>
				</li>
				<li class="nav-item">
					<a class="nav-link" href="#" data-scroll="Skills">Skills</a>
				</li>
				<li class="nav-item">
					<a class="nav-link" href="#"  data-scroll="Portfolio">Portfolio</a>
				</li>
				<li class="nav-item">
					<a class="nav-link" href="#"  data-scroll="Contact">Contact</a>
				</li>
			</ul>

		</div>
	</div>
</nav>

Answer 1

In

$(document).ready(function(){ 
  $('#nav-b .navbar-item li a').click(function(){ 
    $('#nav-b .navbar-item li a').removeClass('active');
    $(this).addClass('active'); 
  }); 
});

У вас нет элемента navbar в html, поэтому попробуйте:

$(document).ready(function(){ 
  $('#nav-b .nav-item li a').click(function(){ 
    $('#nav-b .nav-item li a').removeClass('active');
    $(this).addClass('active'); 
  }); 
});