Bootstrap 4 Cargando Spinner en el botón

Solución:

Debe agregar Bootstrap 4.2.1 para este código mientras usa 4.0

$(document).ready(function() {
    $("#btnFetch").click(function() {
      // disable button
      $(this).prop("disabled", true);
      // add spinner to button
      $(this).html(
        `<span class="spinner-border spinner-border-sm" role="status" aria-hidden="true"></span> Loading...`
      );
    });
});

<link rel="stylesheet" type="text/css" href="https://stackpath.bootstrapcdn.com/bootstrap/4.2.1/css/bootstrap.min.css">

<div style="margin:3em;">
  <form class="form-inline" id="topicForm" action="" method="POST">
    <input type="text" id="inputTopic" name="topic" class="form-control mb-2 mr-sm-2" placeholder="Topic of interest" required autofocus/>
    <button type="button" id="btnFetch" class="btn btn-primary mb-2">Submit</button>
  </form>
</div>

<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.6/umd/popper.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.2.1/js/bootstrap.min.js"></script>

Parece que el CSS giratorio no está presente en la biblioteca CSS cargada. Pruebe los siguientes códigos

En CSS

    @keyframes spinner-border {
      to { transform: rotate(360deg); }
    } 
    .spinner-border{
        display: inline-block;
        width: 2rem;
        height: 2rem;
        vertical-align: text-bottom;
        border: .25em solid currentColor;
        border-right-color: transparent;
        border-radius: 50%;
        -webkit-animation: spinner-border .75s linear infinite;
        animation: spinner-border .75s linear infinite;
    }
    .spinner-border-sm{
        height: 1rem;
        border-width: .2em;
    }

En JS

$(document).ready(function() {
    $("#btnFetch").click(function() {
      $(this).prop("disabled", true);
      $(this).html(
        `<span class="spinner-border spinner-border-sm" role="status" aria-hidden="true"></span>Loadingg...`
      );
    });
});

Pluma de código

¿Podrías intentar agregar [In head] Lo agregué funcionó en su codepen

  <link href="https://getbootstrap.com/docs/4.2/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-GJzZqFGwb1QTTN6wy59ffF1BuGJpLSa9DkKMp0DgiMDm4iYMj70gZWKYbI706tWS" crossorigin="anonymous">

y esto

 <script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>

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