If you mean why was Nora giving apparent reasons for not firing Krogstad, one of the main reasons Nora did not want Krogstad fired is due to the debt she owed him. She was afraid he would share the secret debt that she owed him. Nora was afraid that Krogstad would blackmail her if she did not help him keep his job:
On Christmas Day, Nora is alone, pacing and fretting over Krogstad’s threats. Anticipating shame, and considering abandoning her family, Nora briefly discusses with the nanny, Anne-Marie, the possibility of no longer being around her children. Nora tries to distract herself but remains jumpy, as if Krogstad could appear at any minute to ruin her.
As it turns out, that is exactly what Krogstad did. He threatened Nora. He told her that he had a letter written to Torvald explaining all about the secret loan.
Krogstad threatens Nora with the letter. He tells her he will tell Torvald the truth if Nora does not help him keep his position in the bank. Nora is petrified. She is frozen with fear. She begs Torvald to not fire Krogstad. Of course, Torvald cannot truly understand her fear. When he receives the letter from Krogstad, Torvald loses control. He yells and screams at Nora. She is is in utter shock at his reaction.
Nora had hoped for some miracle to happen. She thought Torvald would sacrifice his reputation for her. When that did not happen, Nora had no choice but to leave Torvald. Krogstad's plan to destroy her happiness worked. Torvald's true character was revealed in the process. No matter how hard Nora tried, her happiness and her world fell apart because of Krogstad's letter. That is why she tried so hard to protect his position in the bank.