Can't purchase photo book on 'Photos.
I have just completed a Mac photo book. However, I was unable to buy it, as a message popped up saying that I could no longer purchase the book and to update 'Photos' before trying to buy again. I am having difficulty working out how to update 'Photos' and am not sure how to proceed. I checked my updates on iTunes & all is up to date. Any help would be gratefully received. Thanks.
MacBook Pro, OS X El Capitan (10.11.6)