This version seems to work

book_a_flight.sql

DROP FUNCTION IF EXISTS reserve_seats;
DELIMITER |
CREATE FUNCTION reserve_seats(passenger_count INT, flight_id INT) RETURNS INT
BEGIN

  DECLARE available_seats INT;
  DECLARE booking_id INT;

  (SELECT (60 - COUNT(*)) INTO available_seats FROM daily_schedule JOIN bookings ON daily_schedule.id = bookings.flight JOIN tickets ON tickets.booking = bookings.id WHERE daily_schedule.id = flight_id AND bookings.payment IS NULL);

  # Make sure flight isn't full
  IF available_seats < passenger_count THEN
    RETURN 0;
  END IF;

  # Reserve seats
  INSERT INTO bookings (flight) VALUES(1);
  (SELECT LAST_INSERT_ID() into booking_id);

  INSERT INTO tickets (booking) VALUES(booking_id);

  RETURN booking_id;

END|
DELIMITER ;