500 Internal Server Error

When I import content from the site library，the third step I get the error “500 Internal Server Error”

The server error log is following
---------------------------------------------

Please install an MTA on this system if you want to use sendmail!
[Tue Jul 24 07:15:49.672318 2018] [:error] [pid 25382] [client 111.176.96.68:53598] PHP Warning: Creating default object from empty value in /var/www/html/wordpress/wp-content/plugins/gp-premium/sites/classes/class-site.php on line 722, referer: http://39.106.141.251/wordpress/wp-admin/themes.php?page=generate-options&area=generate-sites
PHP Fatal error: Uncaught Error: Class 'XMLReader' not found in /var/www/html/wordpress/wp-content/plugins/gp-premium/sites/libs/wxr-importer/WXRImporter.php:124\nStack trace:\n#0 /var/www/html/wordpress/wp-content/plugins/gp-premium/sites/libs/wxr-importer/WXRImporter.php(332): GeneratePress\\WPContentImporter2\\WXRImporter->get_reader('/var/www/html/w...')\n#1 /var/www/html/wordpress/wp-content/plugins/gp-premium/sites/classes/class-site-helper.php(223): GeneratePress\\WPContentImporter2\\WXRImporter->import('/var/www/html/w...')\n#2 /var/www/html/wordpress/wp-content/plugins/gp-premium/sites/classes/class-site.php(781): GeneratePress_Sites_Helper::import_xml('/var/www/html/w...', 'sider')\n#3 /var/www/html/wordpress/wp-includes/class-wp-hook.php(284): GeneratePress_Site->import_content()\n#4 /var/www/html/wordpress/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters('', Array)\n#5 /var/www/html/wordpress/wp-includes/plugin.php(453): WP_Hook->do_action(Array)\n#6 /var/www/html/wordpress/wp-admin/admin-ajax.php(99): do_action('wp in /var/www/html/wordpress/wp-content/plugins/gp-premium/sites/libs/wxr-importer/WXRImporter.php on line 124, referer: http://39.106.141.251/wordpress/wp-admin/themes.php?page=generate-options&area=generate-sites