I still use iPhoto since Photos does not do everything that iPhoto does. So today I upgraded an old MacBook to Yosemite OS X 10.10.4 and iPhoto would not work. It said to search the Mac App Store and download a new version, but no new version was found. Apple dropped it when Photos was released. A little Googlin’ revealed that if you had iPhoto before you could get it from the Purchases tab of the Mac App Store.
Lo and behold, there it was but it had an Open button since iPhoto was already installed. Clicking the Open button did no good and gave me same error. Checking, I found that I had iPhoto 9.6 which had been working with Yosemite just last month. Checking a newer Mac, I found that iPhoto 9.6.1 was working fine with the latest Yosemite so I copied it over to the old MacBook where it worked fine.
I wondered what I would have done if I didn’t have 9.6.1 to replace 9.6. What if I deleted 9.6 before checking the App Store? I did so but there still was an Open button under Purchases because iPhoto was also on the backup drive. Anyway, once I removed all copies of iPhoto from the MacBook, the App Store presented me with an Install button and subsequently installed iPhoto 9.6.1.