doesn’t work after activation

Breadcrumb NavXT doesn’t include a shortcode (and does not advertise having a shortcode). Breadcrumb NavXT does include a widget, an editor block (Gutenberg block), and there are instructions on how to call the breadcrumb trail using the bcn_display() function (see https://mtekk.us/archives/guides/calling-the-breadcrumb-trail/). If you need help with getting Breadcrumb NavXT integrated with your theme, you should open a support ticket: https://wordpress.org/support/plugin/breadcrumb-navxt/