Q: I have created a photo book using the photos app and when i click on 'buy book' the 'place order' button isnt highlighted so i cant order it.. how do i fix this?

does the book preview correctly?  https://support.apple.com/en-us/HT201504

Any error messages or other information?

If the preview is good and nothing else is obvious go to yoru Apple store account and re-enter all of your information and try again

LN

The "Buy Book" page should show a selection of shipping addresses.  Have you selected a shipping address? Only afte you do that, will the Place order button be shown.

Check, if you are connected to the correct Print Products Store for your country in the Photos > preferences > Genra, and then check all settings for your Print Products Store account  in the menu entry "Photos > Print Prducts Store Account".

Try In the Apple store deleting and re-entering all account information  including billing

LN

I had this in the past, turned out my own adres-card in Contacts got somehow corrupted. Deleting that card, sync with iCloud (so it got deleted there as well), then create a new adres card mark it again as "this is me" and the problem was gone. Took me a 45 min. phone-call with Apple support to figure this out.

HtH

Ben