。
She has a large office in this building.
Literal
She [topic-は] this building [in-に] big office [object-を] is-maintaining.
オフィスを構える ('have/maintain an office') uses 構える ('to set up, establish, maintain') in its 'occupy/run an establishment' sense — the same verb covers having a house, a shop, a workshop, or a base of operations. The ~ている locks in the ongoing state of having that office. ビル ('building,' from English) refers specifically to large modern buildings — contrasted with native 建物 ('building' generic) and 家 ('house'). The combination ビルにオフィスを構える is standard for describing a professional with a permanent workspace, often suggesting a degree of established status.