PHP XML-RPC Prerequisites
If you want to use XMLRPC for web services, the PHP XML-RPC libraries must be installed and configured.
If you are installing Moodle for the first time, it will check for the existence of the PHP extension XML-RPC. Please ensure it is installed.
If you have already installed Moodle you will need to ensure that XML-RPC is installed, you can check by using PHP Info found in the Joomla backend under Help / System Info / PHP Information.