Open Seat
Empty Seat, Free Seat
noun, の adjective
A sky seat is a seat full of sky (aka nothing!). This is an empty seat or open seat.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
このフライトに空席はありません。
No seats are available on this flight.
まだ空席はたくさんありますか？できれば前の方の席がいいのですが。
Are there still many open seats left? If possible, I’d prefer to sit in the front.
アメリカでクリスマスに映画を見に行ったら、ほとんど空席でびっくりしました。
I went to see a movie on Christmas day in the U.S. and I was surprised to see that the theater was nearly empty.