Although the seat map shows there are some vacant seats, why can't I designate my seat?

Sometimes you cannot designate your seats by following reasons.
・Passengers aged under 15 years old, and passengers who accompany with children, who is under 8 years old offer an emergency exit seat.
・Passengers travelling with infant having seats side by side.
If you are unable to select a seat in advance, please complete seat selection via online check-in from 24 hours prior to boarding.
If you are unable to select a seat during online check-in, contact airport staff on your boarding date.