Ring Access Controller Pro without Ring Camera/Doorbell

Can someone please explain why a Ring Doorbell or Ring Camera are required to setup the Ring Access Controller Pro in the Ring App? I have this installed on the gates of a small apartment building (2 gates) and I just want to be able to share access control with the tenants. Amazon delivery drivers can let themselves in, but the people who live there can't use it unless there is a Ring Doorbell? What gives?