。
She was always complaining.
Literal
She [topic-は] always complaints nothing-but [object-を] was-saying.
常に (つねに, 'always, constantly') is more literary than いつも. 不平 (ふへい, 'complaint, grievance') with ばかり ('nothing but') emphasizes that complaining was all she did. The ~ていた marks this as a habitual past action — she used to do nothing but complain.