There are a few factors here, but it’s most likely up to the laws in your country, and as far as I know, most countries do allow (or at least not explicitly ban) you to sell AI generated content, but some countries have laws which don’t allow you to claim copyright over that content - meaning that you’re allowed to sell it, but you couldn’t e.g. prevent others from copying your book cover image and using it in their own work.
There are a few factors here, but it’s most likely up to the laws in your country, and as far as I know, most countries do allow (or at least not explicitly ban) you to sell AI generated content, but some countries have laws which don’t allow you to claim copyright over that content - meaning that you’re allowed to sell it, but you couldn’t e.g. prevent others from copying your book cover image and using it in their own work.