If you can't add a G+ page to Social, it could be because it was set up as an individual's profile, not a G+ page. You can't add individual profiles to Social because of an API limitation, and will probably never be able to. If you accidentally made an individual profile, set up a new G+ page to add to Social.
Google tip: If you are unsure whether you’re viewing a page or an individual’s profile, look for the +1 icon at the top of the page.
There is more information on the types of G+ pages available here: https://support.google.com/plus/answer/1710600?hl=en