Bootstrap columns won't go horizontal

So I recently began the personal portfolio project and quickly hit a brick wall. I’ve googled for answers and nothing has fixed it. No matter what I try, including all the device size codes, the columns on my screen just stay verticle no matter how much i min-max my browser. Here’s my code…

<div class="container-fluid">
  <header>
    <div class="row">
      <div class="col-xs-3">Left element</div>
      <div class="col-xs">Right element</div>
    </div>
  </header>
  <body>
  </body>
</div>

and this is how it looks…

Left element
Right element

Okay worked out how to show the code here.

I’d like to add that I’ve linked the CodePen project to Bootstrap 4.

I’ve somehow fixed it. For some reason my project hadn’t loaded bootrap 4.