。
She's indispensable to my office.
Literal
She [topic-は] my office [for-に] [indispensable-なくてはならない] person is.
なくてはならない is the obligation/necessity pattern, often translated 'must.' Built as ない (negative) + ては (conditional) + ならない (won't do) — literally 'if [she] is not [there], it won't do.' Used predicatively, the whole phrase modifies 人 ('person') as a relative clause: 'a person without whom things won't function' = 'an indispensable person.' This is one of the most common ways to express indispensability in Japanese.