Question

To what does macbeth refer when he says the witches put a barren sceptre in my gripe?

Answer 1

As the word barren suggests, Macbeth was either sterile, or his wife likewise. Essentially he was unable to sire a male heir to ascend to the throne after his passing or abdication.

Answer 2

In Shakespeare's "Macbeth," the protagonist makes reference to the witches in his soliloquy, and believes that that his power as a king is worthless. The reason is that he considers that the witches' prophecy about Banquo's children becoming kings is a threat to his crown. As a result, Macbeth feels upset and does not enjoy his position as a king, so he hires murderers to kill Banquo and his entire family.