What is Nora's tragic flaw in "A Doll's House"?
Nora's flaw (until she finally had enough) was her insatiable need to seek acceptance from the outside, instead from herself.
Her entire life she lived pleasing her father to obtain his love and affection; then she did the same with her husband, and with society as a whole. Sure, this was expected of women of her time, but the character she let others make her become was a game of negotiation for approval.
Since a "tragedy" is made of the battles of the protagonist against circumstances then the circumstances which enabled this behavior on Nora's part surely put her on the losing end, considering that women were seen as caricatures, and as "dolls" to be played with.
Hence, when Nora finally noticed that, during all those years of seeking approval, acceptance, and love she was never seen with humanity, her negotiations ended in cold. She literally realized how much of a game she had been playing, and she simply turned around from it all, and made something of herself (or so we hope).
I don't see Nora as a tragic heroine in the classical sense; however, her primary flaw is her deceptiveness. Nora cloaks her every thought and deed in lies, regardless of whether there is any real benefit in the charade. She is almost a compulsive liar. Nora does not hesitate to commit forgery and then lie to Torvald about how she got the money from the loan. She lies to him about household expenses. Deceit appears to drive all of her actions.