。
She lacks common sense.
Literal
She [topic-は] common-sense [in-に] is-lacking.
常識 (じょうしき, 'common sense') is a loaded word in Japanese — it covers not just practical wisdom but social awareness and accepted norms. 欠ける (かける, 'to lack, to be deficient in') takes に for what is lacking. The ~ている form marks this as an ongoing characteristic rather than a temporary state.