where is the "complete my album" feature in the itunes store?

Claire,

Also make sure that the original purchase was on the same Apple ID that you are currently using. But if all that checks out and you still don't see the "complete my album," reach out to Apple Support:

https://getsupport.apple.com

Claire,

If you have bought one or more tracks from that exact album, the Store will offer you the "complete my album" discounted price. However, in some common cases, such as a single that is released separately from the full album, or where an album is replaced by a different (e.g., remastered) version, the Store will not connect your past purchase to the album.

If you have a case where you are sure you are entitled to the "complete my album" price but don't see it in the Store, contact Apple Support.

Maybe not. What happens is iTunes sometimes reissues the same album and for whatever reason gives it a different ID number. It sees it as a different album and will even allow you to purchase the same album twice. If that’s the case, you can’t tell it’s the same album without a lot of digging and trying to find the album ID. I would agree with the suggestion to contact or if it’s even possible call iTunes Store