First, try to open the feed in the browser by clicking the preview icon in Joomla administration Route 66 area. The feed includes the articles created or modified the last hour. If the feed is empty, open and just save a couple of articles and then check the feed again.
When the feed has content it should work. If not please ensure that you have provided the feed URL to the Facebook Instant Articles settings.
We also recommend using the Facebook Pages Manager application on a mobile device to test your Facebook Instant Articles.
Let us know if you need more help.
Regards