This relates to the training/dates of each platinum hostess.
This popped for me upon the FIRST of the three ‘dates’ with the FINAL plantinum hostess “Kirara” who joins you after you beat Sunshine club. I did not have to complete all three dates in order for it to pop, but if you want to access her substory to complete all for the substories achievement, you’ll have to finish training her.
The platinum hostesses who require training/dates are
Koyuki
Kana
Aika
Shoko
Yua
And finally Kirara
Obviously, Yuki does not need training