This error is caused by using iBooks Author's Export functionality and then delivering to the iBookstore. To fix, do the following:
1. Open your book in iBooks Author
2. Choose Publish from the Toolbar or from the File menu
3. Choose a name for your book package
4. iTunes Producer should open automatically once iBooks Author completes the book publishing process.
5. Carefully enter your book's metadata
6. When complete, click on the Delivery tab and click Deliver
User dmingo says this about that:
After audio tracks are placed in your book, you must 'Publish' the book from iBA (using the 'File/Publish...' menu) - that means re-entering all metadata from scratch - you can't simply import the .ibooks file into iTP. If you enter another audio clip at some time, you have to do it all again.