Option to Tell a Friend
CiviCRM is telling you that there is no "supervised" rule configured for the Individual contact type - it doesn't know how to check that those "friends" are new unique contacts and not existing donors/members/contributors etc.

There's some information on deduping and merging contacts in the CiviCRM User Guide

In particular please check that you have a supervised rule configured for individuals as seen in the screenshot on the first linked page.