"A session of a recurring class cannot be deleted: cancel it instead."

Where: Move to trash, on a session.

Why: only a one-off session can be deleted. A session of a recurring class is part of the schedule.

What to do: tap Cancel session (with an optional reason, for example "Public holiday"). To stop the whole class, give it an end date or move it to the trash with Move the class to trash (same details screen).