。
I must not forget to meet her.
Literal
Forget-not-[continuing-で] her [to-に] must-meet.
忘れないで functions as 'without forgetting' — the negative te-form creating an adverbial phrase. ~なければならない expresses obligation ('must'). 逢う uses the kanji 逢 rather than the standard 会 — 逢う often implies a more emotional or fated encounter, giving the sentence a slightly literary or romantic coloring.