iBooks Displaying in Audiobooks Sync not on Mac

I download audiobooks into my iBooks app on my Mac and from there I sync them onto my iPhone 8 (iOS 16.77.11). When performing the sync I receive an error message "Some items were not copied to the iPhone because they could not be found."

The attached screenshot of the syncing operation sows a number of books that are unticked and it's these books that cannot be found. Is it possible to remove these books from the display? If so, please guide me in removing/deleting them?

Note: I have searched my Mac for these books, but none of the unticked books can be found.

[Edited by Moderator]