While your site is publicly accessible, the app cannot access your site’s XML-RPC file: https://holdfastmac.asn.au/xmlrpc.php
This file is used by the WordPress app, and other plugins and apps, to connect to your site. It must return the following message:
XML-RPC server accepts POST requests only.
You can see a working example here:
https://www.ads-software.com/xmlrpc.php
Since your site returns a 404 error for this page, I would recommend that you check the following:
1) Can you see the /xmlrpc.php file at the root of your WordPress installation when accessing your site via FTP? If you don’t, then please try re-installing WordPress. You can do this by going to the Dashboard -> Upgrades page, then click the “Reinstall WordPress” button.
2) Do you use any security plugins that may block access to this file? If so, could you try to disable them? You might also want to check your site’s .htaccess file for any rules that might be blocking access to the xmlrpc.php file.
3) Does your hosting provider block access to this file? If you don’t find any plugin that may block access to the file on your site, I would recommend that you get in touch with your host.