Macbeth is saying that there is no power in his hand
Explanation:
Macbeth knows the prophecy and by know would have interpreted it right in knowing that the death of Fleance was important if he ever had to survive as king because he himself is destined to be a king.
Now, he knows that the escape means that he will have a reason to fight him which would be to avenge his father and also to claim the throne.
As the prophecy has till now been true he knows that his time may just be up now.
