_notifications.html.erb

<% flash.each do |type, message| %>
  <% if type == 'success' || type == 'notice' %>
    <script type="text/javascript">
    $(function(){
      PNotify.success({
        text: "<%= message %>",
        delay: 3000,
        styling: 'bootstrap4'
      });
    });
    </script>
  <% else %>
    <script type="text/javascript">
    $(function(){
      PNotify.<%= type %>({
        text: "<%= message %>",
        delay: 3000,
        styling: 'bootstrap4'
      });
    });
    </script>
  <% end %>
<% end %>

add_to_yarn

yarn add pnotify

application.js

** resto do teu arquivo** add essa linha
//= require pnotify/dist/umd/PNotify

application.scss

@import "pnotify/dist/PNotifyBrightTheme";
@import "custom_pnotify";

custom_pnotify.scss

.ui-pnotify {
  margin-top: 50px;
}