では言い表せません

cannot be expressed in ...

→ で
phrase Rare #45,560
cannot be expressed in ... The subject and the means of expression are usually understood from context; for example, 言葉では言い表せません means “cannot be expressed in words.” This is the polite form.
JA: この感謝の気持ちは言葉では言い表せません。
EN: I can’t express this gratitude in words.