justify-content-md-center not working?

I'm following a tutorial and I want to use justify-content-md-center when it reaches medium screen size. But it doesnt seem to work.

 <div class="container">
    <div class="row" style="background:lightblue">
      <div class="col-md-2">col-sm-6</div>
      <div class="col-md-8">col-sm-6</div>
      <div class="col-md-2">col-sm-6</div>
    </div>
      <div class="row justify-content-md-center">
        <div class="col-md-2">col-sm-2</div>
        <div class="col-md-2">col-sm-2</div>
        <div class="col-md-2">col-sm-2</div>
      </div>

  </div>

I appreciate the help thanks