It will add itself to the list to buy once all other criteria is met. check your task list for Amy (spanking connoisseur). you may need to advance Debbie's story first.
I cannot advance Debbie's story cause for that i need to advance the main quest "Many Hands For Many Bottoms" and for that i need to advance Amy's quest " The Spanking connoisseur" and "Teaching Miss Potts"(for this i also need Amy). I'm on 9 step for Amy's quest and it says: "Catch Amy misbehaving and sentence her to private punishment", i cannot cath like for more than 50+ days, all i can is to see her at class without her tie and then punich for that but i cannot lvl up her here, i read the hint at right top corner and it says: pass the rule allowing you to punish her yourself(i can do this) and purchase the chastity underwear from the online store(that i don't have there). Then wait for her to be late for class again( i quess she's not gonna be late till i buy that underwear).