A change in the law is threatening online sales in e-commerce.
Literal
Law [possessive-の] change [because-of-で], electronic-commerce [in-における] online sales [subject-が] being-threatened
における is a formal/written compound meaning 'in, at, concerning' — more formal than bare で. Built from に + おく (連用形 おき) + て (contracted). Common in academic and legal writing. 脅かされている is the passive + ongoing-state form of 脅かす ('to threaten').