What is Hamlet's hamartia?

I concur with the other educators’ responses that Hamlet’s hamartia is most likely his inability to act. He spends far too long pondering whether he should kill his uncle, using various methods including pretending to be mad—although it is debatable whether Hamlet is really pretending—and a play just to catch Claudius and confirm what Hamlet’s father’s ghost revealed.
There is another possibility for Hamlet’s hamartia, though. One could argue that Hamlet’s narcissism is his true downfall. He is most concerned with how his actions will affect chiefly himself. He uses people like Ophelia as pawns in his revenge plot without regard for their feelings. In each of his soliloquies, Hamlet is focused on himself.
So, while I agree that his hamartia is his indecisiveness, it is also his self-centeredness.

Hamlet's hamartia or flaw is most often understood as his indecisiveness or inability to get himself to act to avenge his father's death. This could also be framed as overthinking a problem or avoidance of responsibility.
To some extent, Hamlet is acting reasonably in not rushing off to kill his uncle on the word of a ghost. He is prudent to verify his information, especially as a person's life is at stake. However, once he does verify that Claudius is the murderer, he still hesitates to act, and when he does act, he acts rashly and kills the wrong person.
Even Hamlet chides himself as the play goes on for his paralysis by saying that the pale cast of thought destroys his resolve to do what he needs to do. He simply does not want to murder, and he is filled with anger, which he turns inward to depression and suicidal ideation. We can understand not wanting a responsibility thrust upon us, but Hamlet's hesitation leads to more deaths, including his own.

Hamartia, also referred to as a tragic flaw, is a personal error in a protagonist's personality which results in their unfortunate downfall. One could consider Hamlet's indecisiveness and inability to act upon his instincts to be his hamartia. Following Hamlet's interaction with his father's ghost, he begins to contemplate his revenge. However, Hamlet is a sensitive, insightful, perceptive individual, who struggles to act violently. Despite the fact that Hamlet utterly detests Claudius—and his mother's decision to marry his father's killer—he cannot bring himself to murder Claudius. Unlike his foil Fortinbras, Hamlet delays taking action until he can prove that Claudius murdered his father. Even after witnessing Claudius' reaction to the play, Hamlet does not murder him while he is praying. Hamlet talks himself out of killing Claudius by saying,

"Now he is a-praying. And now I’ll do ’t. And so he goes to heaven. And so am I revenged.—That would be scanned. A villain kills my father, and, for that, I, his sole son, do this same villain send To heaven" (Shakespeare, 3.3.69-80).

Hamlet also contemplates committing suicide several times but decides against it out of fear that his soul would be doomed. Hamlet's indecisiveness—directly and indirectly—leads to the deaths of Ophelia, Laertes, Gertrude, and Claudius. In the end, Hamlet's revenge does not go as planned, and nearly all the important characters in the play tragically die. The audience can sympathize with Hamlet's hamartia, which makes him one of the most well-rounded, enigmatic, and complex characters in all of literature.
https://literarydevices.net/hamartia/