when I use the Code

<div class = "container">
   <nav class = "navbar navbar-expand-lg navbar-dark indigo">
     <a class="navbar-brand" href="#"> Navbar </a>
   </nav>
</div>

when I add fixed-top

<div class = "container">
         <nav class = "navbar fixed-top navbar-expand-lg navbar-dark indigo">
             <a class="navbar-brand" href="#"> Navbar </a>
         </nav>
     </div>

Fixed-top disables the container class.