"This subscription type is used by students: change their subscription before removing it."

Where: Subscription types > a type's sheet > Move to trash (owner only).

Why: at least one student has a current or upcoming subscription of this type. These do not block it: a subscription whose end date has passed, an expired card, a used-up card. The type's card shows how many students use it (“Used by N students”).

What to do: on each affected student's profile, Subscriptions block: Edit to give the subscription an end date, or Move to trash and Add a subscription of another type. Once no current subscription uses the type, it can be moved to the trash.