What does dr jekyll fail to understand?

Dr. Jekyll fails to understand the duality of human nature and the inherent conflict between his good and evil impulses. He believes he can separate his darker side, Mr. Hyde, from his respectable persona without facing the consequences of such a division. This misunderstanding leads to his ultimate downfall, as he underestimates the power of his darker impulses and their ability to overtake him. His tragic flaw lies in his hubris, believing he can control the uncontrollable aspects of his own identity.