Pronounce the first character as kòng (4th tone, meaning empty/vacant), not kōng (1st tone, meaning sky/air). Compare: 空缺 (kòngquē, vacancy) vs 空气 (kōngqì, air).
usage
填补空缺 (tiánbǔ kòngquē) means "to fill a vacancy." Used for jobs, positions, and figuratively for emotional gaps.