You would not be able to add the website due to following reasons.

Problem 1: HTTP Loopback - HTTP loopback is a condition where the server couldn't access the site which is hosted in the same server. The server can't accept calls pointed towards the same server.

Solution: It is recommend that you change the host of InfiniteWP panel. We would suggest you to go for a VPS from InMotion Web Hosting for trouble free functionality of your IWP Admin Panel. 

(this recommendation is based on feedbacks and reviews from our long standing users)

Problem 2: Firewall - Your WP site's firewall rule may be preventing calls from your InfiniteWP server.

Solution: Whitelist the IP of the InfiniteWP server on your WP site.

Problem 3: 402 / 403 / 404 errors or the like

Solution: See image below