Photo Book error message

Having created a photo book, I clicked the "Buy Book" button at the bottom and got the following error message:

Your book seems to have frames on one or more pages that do not contain photos. You must either change the layout of those pages or place photos in those frames before you can proceed with this book.

All frames DO have photos in them, and the layout seems correct. One page has a yellow triangle with an exclamation point at the bottom, so perhaps it is flagging that page. But I cannot understand why. It is just a single photo.

Any help would be much appreciated as this is an important gift for my mother.