Yes, you can use the image created by Copilot for your book cover, even commercially, as long as you comply with Microsoft’s terms of use for generated content.
Here’s the key point: Under the Microsoft Copilot and Designer terms, you are granted broad rights to use, publish, and distribute the images you create, including for commercial purposes like book covers. Microsoft does not claim ownership of the generated image—you do. However, you are responsible for ensuring that the image does not infringe on third-party rights (for example, if your prompt included trademarked logos or celebrity likenesses).
Here's the link - https://designer.microsoft.com/termsOfUse.pdf