Question

When Huck is telling Mary Jane about his plan to unmask the phony "uncles," why can't he tell her where he has put her stolen money?

Answer 1

Huck cannot reveal the secret to Mary Jane because he hid the money inside her father's coffin.
When the phony 'uncles' stole the money from Mary Jane and her family, Huck felt bad for them, so he stole the money from the 'uncles' and stashed it away inside the coffin of a dead man, a day before his funeral. Obviously, he couldn't just let her know where he hid it, so had to find other ways to reveal this secret and make everyone happy.

Answer 2

Huck can't tell Mary Jane where he has put her stolen money because he has hidden it somewhere no one can get it -- in her uncle Peter's coffin.

To make matters worse, before he can get the money, the coffin is sealed shut. As a result, when Huck is telling Mary Jane about his plan to unmask the phony "uncles," he does not mention where he hid the stolen money.