Portfolio Help with Nav Bar populating icon in small device

I want to thank everyone who has posted projects, requested help and those who have helped for assiting me with my project. My navbar does not populate the icon when small screen (using bootstrap 4 – here is m nav code: Your Assistance in finding my mistake would be greatly appreciated.

<div class="navbar-header">
<button class="navbar-toggler navbar-toggler-right" type="button" data-toggle="collapse" data-target="#navbarMjbs" aria-controls="navbarMjbs" aria-expanded="false" aria-label="toggle navigation">
  <span class="navbar-toggler-icon"></span>
   <a class="navbar-brand" href="#">MJBS</a>
  <div class="collapse navbar-collapse justify-content-center" id="navbarMjbs">
  </div>