The phrase "an unoccupied" is correct and usable in written English.
It can be used to describe a space or position that is not currently filled or occupied.
Example: "The real estate agent showed us an unoccupied apartment that was available for rent."
Alternatives: "a vacant" or "an empty".
But there was an unoccupied seat next to me.
Guards barked to block them from an unoccupied V.I.P. area.
"This was an unoccupied building," said a former fire official.
All students should have an opportunity to study at an unoccupied seat.
The State's evidence at trial suggested that Evans had burned down an unoccupied house.
No one really cared, but we did finally get an unoccupied room.
Heidi and I found an unoccupied table along the periphery, which was isolated and dark.
Spectator Konda Walker Chambers's car hit an unoccupied police motorcycle before careering into the cheering throng.
You're on your own, left to wander the aisles in search of an unoccupied staff person.
Later in the morning, 11 protesters were arrested for blocking the street outside an unoccupied building.
"There was an unoccupied seat next to her, so I thought I'd go talk to her".
