You are aware, that you will not be able to order the Apple Photo Book for printing from Apple, once you have finished the book? Apple stopped all print services after September 30, 2018.
You can order books and other print products, if you are upgrading your Mac to macOS 10.13 High Sierra. Then you can use third party project extensions to create and order books. See these user tips:
An exclamation point usually means that the resolution of one item on the page is poor or that the original image file of a photo on the page is missing.
Look at the photos on the pages with an exclamation point and try to find the photos in your library. Check the resolution of the photos (width and height in pixels). If the photos do not have at least 200 pixels per inch for the selected photo place holder, move them to a smaller photo box. To print a photo at at 6x4 inches, it should at least have 1200x800 pixels. also try to export the unmodified original of the photo. If you get an error message, that the original is missing, you have to recover the original from your backup and reimport it to Photos.