Promo codes allow readers to download free copies of your book for promotional purposes. You have access to 250 promo codes which you can distribute to reviewers, bloggers, and other media contacts. Readers will be able to download the book in any territory where it’s available for sale, even before the release date.
See Promo Codes in the Apple Books Publisher User Guide for more information.
Make sure the account is tied to the latest agreement. The Promo Codes button will only display for books with a status of On Store or Ready for Store in at least one territory. Additionally, only users with the Admin, Legal, or Sales roles can request promo codes.