Apple stopped selling iPhoto with the release of MacOS X 10.10.3 and the new Photos.app. You can neither update nor buy iPhoto any longer, but you can reinstall it, if you previously bought iPhoto from the AppStore or updated it using the AppStore. See this User Tip on how to update by reinstalling:
Get iPhoto 9.6.1 if you didn't update before OS... | Apple Support Communities
The main idea is to move the incompatible iPhoto app to the Trash, so Spotlight does not see it as installed, reload the App Store by pressing ⌘R, and then to try to download again from the App Store - from your Purchases tab, not from the main page, since you can neither buy iPhoto nor update it, but you can reinstall, if your AppleID is associated with it.
So move iPhoto to the Trash (don't empty the Trash), relaunch the App Store. The button to the right if iPhoto should change to Install on the "Purchases" tab.
If you do not see iPhoto there, try to unhide your Purchases as described on this page: Hide and unhide purchases in the Mac App Store - Apple Support