Connecting Your Site
You must register your site URL with your Facebook page before you can begin publishing Instant Articles.
Go to the Instant Articles Configuration screen on your Facebook page, scroll down to the Connect Your Site section and copy the Page ID. Paste the ID you just copied into the Authorization ID field on the General Settings page of the plugin Settings screen and save it.
Copy your site URL and paste it into the URL input in the Connect Your Site section and click Claim URL.
If you successfully claimed your URL then it should appear under the Your Registered URLs section.