Looks like RSS Ground can't access XML-RPC file in your blog.

Could you please try using RSS Ground plugin for Wordpress posting.

If you have already installed RSS Ground plugin and still can't validate your blog, most likely one of your security plugins or your hosting provider blocks access to xmlrpc.php.

Please, try deactivating your security plugins and check Cloudflare settings (in case you use it). If you still get the error, you need to whitelist our IP: 109.205.61.166 in your hosting firewall settings or contact your hosting company and ask to do so.