。
She parked in an empty lot.
Literal
She [topic-は] empty-lot [at-に] parked.
空き地 (あきち) literally means 'empty land' — the 空き (empty) prefix is built from 空く ('to become empty/vacant') and is productive: 空き家 (vacant house), 空き缶 (empty can), 空き時間 (free time / open slot in one's schedule). 駐車する uses the goal-marker に for the resting location — distinct from で, which would mark the location of an action. Parking is the result of arriving and stopping somewhere, so に fits.