Lightning widget is not visible

In OpenCart 3, first of all try to Refresh theme cache:

Next, take a note that Lightning widget is not visible to users, that do not have rights to modify store settings (setting/setting).

If your admin is modified to use another template, Lightning may not be able to show the widget.

With Lightning 3.09 and higher you can access Lightning widget directly by URL: http://myshop.com/?lightning or by clicking the modification title: