DOMDocument error when generating static site

I get the following error message when I try to generate my static site:

Error: (1) Class ‘DOMDocument’ not found in /var/www/html/styleguide/wp-content/plugins/simply-static/includes/class-simply-static-url-extractor.php on line 132

The file class-simply-static-url-extractor.php is present and executable in the plugin folder. I don’t know how to debug this.

https://www.ads-software.com/plugins/simply-static/