bootstrap panel not working

If you are using Bootstrap 4 read this:

Panels, thumbnails, and wells
Dropped entirely for the new card component.

Panels
.panel to .card, now built with flexbox.
.panel-default removed and no replacement.
.panel-group removed and no replacement. .card-group is not a replacement, it is different.
.panel-heading to .card-header
.panel-title to .card-title. Depending on the desired look, you may also want to use heading elements or classes (e.g. <h3>, .h3) or bold elements or classes (e.g. <strong>, <b>, .font-weight-bold). Note that .card-title, while similarly named, produces a different look than .panel-title.
.panel-body to .card-body
.panel-footer to .card-footer
.panel-primary to .card-primary and .card-inverse (or use .bg-primary on .card-header)
.panel-success to .card-success and .card-inverse (or use .bg-success on .card-header)
.panel-info to .card-info and .card-inverse (or use .bg-info on .card-header)
.panel-warning to .card-warning and .card-inverse (or use .bg-warning on .card-header)
.panel-danger to .card-danger and .card-inverse (or use .bg-danger on .card-header)

https://v4-alpha.getbootstrap.com/migration/#panels-thumbnails-and-wells

For Bootstrap 4, panels have been dropped in favour of cards

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/css/bootstrap.min.css" rel="stylesheet"/>


<div class="card" style="width: 20rem;">
  <div class="card-body">
    <h4 class="card-title">Card title</h4>
    <h6 class="card-subtitle mb-2 text-muted">Card subtitle</h6>
    <p class="card-text">Some quick example text to build on the card title and make up the bulk of the card's content.</p>
    <a href="#" class="card-link">Card link</a>
    <a href="#" class="card-link">Another link</a>
  </div>
</div>