They hit your Daily Order Limit: the number of catering orders already booked whose event time falls inside the Hours Before/Hours After window around their chosen time has reached your Max Number Of Orders setting. They need to pick a different event time (earlier or later) on that date, or you can raise the limit / widen or narrow the before/after window in Catering Settings. This check only runs when the limit toggle is turned on and the max-orders value is greater than 0.