Is there a guide on how to use this plugin?

I am trying to generate a sitemap.xml but the file doesn’t show up after I save settings.

Here is an error I found in the apache log:

[Sat Feb 24 17:55:11.743583 2018] [:error] [pid 27207] [client 192.168.0.101:49090] PHP Fatal error: Uncaught Error: Class ‘DOMDocument’ not found in /var/www/www4.example.com/wp-content/plugins/lana-sitemap/includes/class-lana-sitemap.php:662\nStack trace:\n#0 /var/www/www4.example.com/wp-content/plugins/lana-sitemap/includes/feed-sitemap-news.php(15): Lana_Sitemap->xml_document(‘news’)\n#1 /var/www/www4.example.com/wp-includes/template.php(688): require_once(‘/var/www/www4.d…’)\n#2 /var/www/www4.example.com/wp-content/plugins/lana-sitemap/includes/class-lana-sitemap.php(1351): load_template(‘/var/www/www4.d…’)\n#3 /var/www/www4.example.com/wp-includes/class-wp-hook.php(288): Lana_Sitemap->load_template_news(false)\n#4 /var/www/www4.example.com/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(”, Array)\n#5 /var/www/www4.example.com/wp-includes/plugin.php(453): WP_Hook->do_action(Array)\n#6 /var/www/www4.example.com/wp-includes/functions.php(1245): do_action(‘do_feed_sitemap…’, false, ‘sitemap-news’)\n#7 /var/www/www4.example.com/wp-includes/template-loader.php(37): do_feed()\n#8 /va in /var/www/www4.example.com/wp-content/plugins/lana-sitemap/includes/class-lana-sitemap.php on line 662, referer: https://www4.example.com/wp-admin/options-reading.php