。
She tried to break her child's habit of lying.
Literal
She [topic-は] child [genitive-の] lie tell habit [object-を] fix-tried-to.
Two layered constructions. うそをつく ('to tell a lie') is a fixed collocation; the verb つく here is the same as in the everyday 嘘つき ('liar'). The whole phrase うそをつく modifies the noun くせ ('habit, mannerism, ingrained tendency') as a relative clause — 'the habit of telling lies.' 直す ('to fix, repair, mend, correct') is widely used for fixing physical things, errors, or habits. The construction 直そうとした uses the volitional + とする pattern to express an attempt: 'tried to fix.'