“4. Check your XMLRPC file. You can do this by going to your web site URL and adding “/xmlrpc.php” to the end (without the quotes). When it loads in your browser, you should see “XML-RPC server accepts POST requests only.” on a line by itself.
If you see this message, but it is not on a line by itself, a theme or plugin is displaying extra characters when it shouldn’t. See points 2 and 3 above for debugging steps.
If you get an 404 Error Not Found message, contact your web host. They may have security in place that is blocking XML-RPC.”
That is what written on the support page of Jetpack: