Because she was mistakenly inserting the earphones instead of the smartphone charger.
Because she was mistakenly pointing at the earphones instead of the smartphone charger.
さす (入れる)= to insert (into)
彼女はイヤホンをさしていた = she was inserting the earphones
間違えて = mistakenly
スマホの充電器 = smartphone charger
なぜなら、彼女は間違えてスマホの充電器と間違えてイヤホンをさしていたから
Because, she was mistakenly inserting the earphones instead of the smartphone charger.
指す (指で) = to point at
なぜなら、彼女は間違えてスマホの充電器と間違えてイヤホンを指していたから
Because she was mistakenly pointing at the earphones instead of the smartphone charger.
Because she accidentally plugged in her earphones instead of her phone charger.
・Because she accidentally plugged in her earphones instead of her phone charger.
plug in(コンセントや端末にプラグを挿す)という動詞を使って、accidentally(うっかり・間違えて)と instead of her phone charger(スマホの充電器の代わりに)を組み合わせることで伝わります。
・That's because she mistakenly plugged her earphones in thinking it was her phone charger.
「なぜなら、彼女はスマホの充電器だと思って間違えてイヤホンを挿していたからです」という意味の表現です。