You may see this message when trying to add an Express Photobook to your shopping cart. Express Photobooks have several 'picture holders' for your photos. You cannot add a book to cart when one of these is empty. You should either fill in all the picture holders with photos from your album or you should delete the picture holder.
To find the picture holder that is empty, click the Back button and then the 'Previous' or 'Next' buttons to flip through each page. The photo holders can be identified by the grey colored areas with the message 'Drag a photo here'. If you see this message then you need to either drag a photo on to it to remove this message or you need to delete the holder. To delete it - simply click on the area and press the 'Delete' button.
Once all these picture holders have been filled (or deleted) you can try to add it to your cart again. Remember that you may need to look carefully - some holders may be really small and can be easily missed.