URL is not Being Accepted?
Your Website URL is not being accepted by Facebook Instant Articles because Your website has very less traffic from facebook. Same happened with me. I have the solution. You need to have at least 15000 page views of your website. These views should be from links shared on Facebook so that Facebook understands that your website is being opened by people using facebook when they click on link shared of your website. Try this method. It will surely work.
As per the new policy update the website need a minimum amount of traffic to apply for new instant article or to claim new domain on allready approved pages. Dont know how much page views or traffic need.
Things to Do:
Publish few articles on your website and share the content (Web Link) on facebook and increase traffic.