Hi! Erika is the first S+ girl you recruit in the Hostess Club, you need to buy it on the shop for 5k coins and then you can start recruiting girls
I think there may be a bug in obtaining her, then. I started playing in one of the older versions and never obtained her, nor did she ever appear in the shop despite me owning the Hostess Club. Currently on the most modern version for Itch and her name is still greyed out "???" and her purple heart on the app is ?.