I solved this just now for myself and ordered a photo book.
My search shows this problem has been around for years.
Problem: Select BUY BOOK in Aperture to buy a photo book you have created in aperture....
Then a window pops up to enter your Apple ID ....(in my case, aperture is already logged in to iCloud and handling all my photo stream daily!)
I would enter my apple ID and PW (which is universal for all Apple stuff from my experience....Store, App Store, iTunes, Applecare, etc.)...
then select login or existing ....THEN YOU GET THIS PAGE WANTING YOU TO CREATE AN APPLE ACCOUNT !!!!
WEIRD! I went to store, updated credit card, zip code (because some posts proposed that was a problems, etc
Finally, Just CONTINUED TO ENTER MY OLD PASSWORD IN THIS create account form, next page credit card, etc.....
It took it all (without saying, "HEY, this account already exists", as you would expect)
Placed my book order.
Now I just tried buying another book....no problem, don't have to go through this again!
I suspect a third party vendor for the books is the reason...just a guess. But Apple should still have it fixed by now. (scold scold)