In older versions of WordPress.org sites, XML-RPC is not enabled (XML-RPC is enabled by default on WordPress.com sites).
In order for your WordPress installation to be able to serve XML-RPC requests you have to enable this feature on you website. To enable this feature, login to your WordPress site and go to the admin section. There, under Settings -> Writing you will have to Enable the XML-RPC option as shown below. Put a check next to this option and you’re all set.