I figured it out!
I was getting this same error an getting very frustrated about it.
The solution that worked for me was very simple: Change the rss settings (under Setings–>Reading) to “Full Post” (instead of “Summary”). Once I made the change, I went back to Instant Articles for WP, clicked “Submit for Review” and the button worked as intended.
Then, I went to my Facebook page Instant Articles settings and, yep, i got the message that I’m pending review.
Hello, I can fix this in the advanced configuration in facebook app
https://developers.facebook.com/apps/ [Your APP ID] /settings/advanced/
I’ve added the exactly URL of the domain there, in the Domain Manager.
hope this help