If it is actually the class `DOMDocument` that is missing, you may have to check your installation of PHP. The DOM module should be included by default though.

If you have command line access on the server you can check which modules are installed by running `php -m`. You are looking for the one called `dom`. Note that libxml (and thus the xml module) are a requirement.

If you find that either the dom or xml module is lacking, depending on your system, your package manager may offer a package for `php-xml` that includes both.