Why is Krogstad's attempt at blackmail a successful one?

Krogstad's attempted blackmail is successful because he knows of Nora's secret loan and fraud, which could ruin her reputation and marriage if exposed. Additionally, he is aware of Torvald's professional position and influence, making Nora vulnerable to his threats. Krogstad's control over this damaging information gives him power over Nora, enabling him to manipulate her.