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She's not in the mood to do that work.
Literal
She [topic-は] that work [object-を] do mood [topic-は] not.
~気分ではない ('not in the mood to') uses 気分 ('mood, feeling') in its predicative sense — 'is not the mood [of doing X].' The では here is the negative copula form (で + は), with は adding contrastive emphasis. Compare with 気が進まない ('not feeling like, reluctant'), ~したくない ('don't want to do'), ~する気がしない ('don't feel like doing'). Each captures a different shade of disinclination; ~気分ではない leans toward 'I'm not in the right headspace' rather than dislike or refusal.