WP Page Builder – not functional

I can’t seem to get any drag-and-drop page builder working. I tried Elementor and now WP Page Builder. I selected the correct template during configuration, but when I try to “Edit with WP Page Builder” the only thing that appears on the screen is a box filled with a scaled down version of the page being edited. I was expecting to see a graphical editor page that shows controls/widgets in a left column for drag-and-drop, but absolutely nothing pertaining to a drag-and-drop editor is shown. No warnings or errors are displayed and the plugin is installed correctly. This is for WP 5.0.4 installed on Debian Linux.

I’d like to give WordPress a fair shake before giving up, but this is getting truly exasperating. Software which depends on installable components needs to alert the user somehow of installation and/or runtime problems. At least with html and javascript I can quickly determine the cause of the problem most of the time.