"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, .panel-success, .panel-info, .panel-warning, and .panel-danger have been dropped for .bg-, .text-, and .border utilities generated from our $theme-colors Sass map.
Laravel 5.6 comes out of the box with the default Boootstrap version being the newest one, which is 4.1.
Panels have been replaced by the Card module in the newest version of Bootstrap so using panel won't result in anything happening unless of course you have your own custom 'panel' class.
So you can either down grade back to Bootstrap 3, or you can jump into Bootstrap 4 starting here: