"This subscription has no price: add a price to the plan or to the student."

Where: Fees screen or student profile, when confirming a payment or correcting a due date on a subscription without a price.

Why: without a price, no due date can be calculated, so the payment cannot be applied.

What to do: the owner adds a price to the subscription plan, or a price specific to the student in the profile (Edit the subscription), then confirms the payment again.