必要 + が

The na-adjective 必要 ('necessary, needed') takes the thing that is needed marked with が, not を — 'X が必要だ' = 'X is necessary / X is needed.' バットが必要だ ('a bat is needed'), 水が必要です ('water is required'). Part of the family of adjective-like predicates (好き, 上手, 分かる, できる, 欲しい) that mark their logical objects with が rather than を.