Lady Macbeth fears that her husband is too kind, hesitant, and morally aware to commit murder and seize the crown. She worries he may lack the ruthlessness needed to follow through, so she pushes and manipulates him into acting.

In the play

She sees Macbeth’s conscience as a weakness. Instead of trusting him to act boldly, she fears he will freeze, back out, or be stopped by guilt.

Simple answer

If you want the shortest version: she fears he is not ruthless enough to become king.