"Invalid" when attempting domain mapping

I'm getting an "invalid" when attempting to map a domain to a subdomain

What I have:

- dedicated IP IP and dedicated server with site5.com
- subdomain multisite configuration on mainsite.com
- both mainsite.com and newsite.com's GoDaddy nameservers are pointed to site5.com
- I want newsite.com to be mapped to newsite.mainsite.com
- A records in site5.com (separate cpanels of the two sites) of newsite.com shows the correct IP address
- Originally created newsite.com as a "site" in Multi-admin on Site5 to have MX records in the same place as my other sites
- 'define ('noblogredirect')' to properly redirect mainsite.com to http://www.mainsite.com

I originally decided to set up newsite.com as its own WP install until I realized I could map it with your plugin off my mainsite.com Multisite. So, newsite.com currently has its own WP install and cpanel. I can completely delete this if necessary but ideally would like to keep the separate cpanel for the domain to keep the MX records all under one roof.

hoping this easy to resolve! Thanks!