What does lady Macbeth fear about her husband

Lady Macbeth fears that her husband does not possess the personality traits that are necessary to achieve his main goal: to ascend to the throne. In other words, she is worried that her husband is too kind to do what it takes to become king, which is murdering Duncan. Moreover, in the play, she is the one that convinces Macbeth to do whatever is necessary to take the throne. Lady Macbeth is also the one that plans Duncan's death and calls her husband 'a coward' when Macbeth tells her that he should not kill Duncan.

Answer: That he won't be strong enough to kill the king and further his own ambitions