Fatal error: Class 'DOMDocument' not found, in a Wordpress site
DOM is PHP module. This error usually occur when we not have this php module installed. Dom listed in phpinfoyou can get this information by running phpinfo() function on any php file.
There are few way to debug and fix this issue. - first install php-xml package
sudo apt-get install php-xmlthen after restarting your apache server
sudo service apache restartIf still this issue is not fixed. check your php version and install appropriate php-xml module for itfor example: for php 7.0
apt-get install php7.0-xml