The Certify tab is used by learners to join their certification sessions.
The 'Coach' tab is a learner-facing feature that allows them to join a live certification with a coach. Learn more about creating Certification cases here.
Coaches and Admins also see the tab (see example below), but when they click on it, it assumes they are a learner:
For coaches to initiate a Certification session, they need to use the 'Coach' tab. See this article on facilitating Certification sessions.