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She doesn't realize how beautiful she is.
Literal
She [topic-は] her own beauty [to-に] is-not-realizing.
気づく ('to notice, to realize, to become aware of') takes its target with に, not を — 'to notice X' uses Xに気づく. With ~ていない (the resultative ~ている negated), 気づいていない reads 'has not (yet) noticed' = 'is unaware of.' 自分の美しさに気づいていない is a familiar literary trope: an oblivious heroine unaware of her own beauty, a setup that lets a narrator or another character do the noticing.