Why this happens
SwingMatch shows real-time availability, but if two students are looking at the same slot at the same time, the first one to complete the booking gets it. By the time you reach the confirmation step, the slot may already be taken.
What to do
Go back and pick a different time. The calendar will refresh to show the currently available slots.
Message the instructor. If no other times work for you, tap the Chat button on the instructor's profile to ask about other availability. When no slots are available, a message button appears with a pre-filled message: "Hi! I'd like to book an [offering] but your calendar is fully booked. Do you have any openings?"
Good to know
This is most common with popular instructors or group lessons with limited capacity.
The booking page shows a red alert: "Timeslot is not available anymore" if the slot was taken after you selected it.
You are never charged for a slot that becomes unavailable. If you entered credit card details, no authorization is placed until the booking is successfully submitted.