jQuery is not defined

Please add jQuery as a dependency to wp_enqueue_script

// set js file
                if ($this->get_opt('use_js'))
                    wp_enqueue_script('wp-external-links', plugins_url('js/wp-external-links.js', WP_EXTERNAL_LINKS_FILE), array('jquery'), WP_EXTERNAL_LINKS_VERSION, (bool) $this->get_opt('load_in_footer'));

https://ww.wp.xz.cn/plugins/wp-external-links/