If it was something like "An unknown error occurred, please try again later", that can mean many things:
- Your book may contain photos with unprintable image files.
- iPhoto may not be able to connect to the store because of network problems or firewalls, anti virus software.
- Your AppleID may not be valid at the Print store you selected.
Unprintable image files: Have you checked, if the book is o.k.? That is, if it does not contain any file that cannot be rendered?
iPhoto, Aperture: Previewing an order in iPhoto or Aperture
Could you preview the book or print it to pdf? Sometimes the upload may be hanging, if one of your image files cannot be processed properly. Removing that image from the book and replacing it by another one usually helps. This can happen if the original master image file is corrupted or the filename of the image is too long or contains ctrl-characters.
Network: Temporarily turn off any firewalls, Antivirus software. Try to boot into safe Mode and to upload the book again:
See: Mac OS X: What is Safe Boot, Safe Mode?
AppleID and Print Store:
Set the "Print Products Store" in the iPhoto Preferences panel > Advanced to the country of your billing address in your AppleID settings. The credit card used with your AppleID needs to be valid in that county. You may need to check your Password and Security question on "MyAppleID":
https://appleid.apple.com/cgi-bin/WebObjects/MyAppleId.woa/
Your password and answers to security questions need to be obey apple's new rules for safety.
Regards
Léonie