It is six years too late to get iPhoto for Yosemite. Apple stopped the development of iPhoto in 2015 and pulled it from the App Store. It has been replaced by Photos for Mac, which is a part of the system.On macOS 10.10.3 or newer you need iPhoto 9.6.1, the compatibility update for Yosemite. If your current version of iPhoto is in your Purchases at the AppStore, you may still be able to reinstall iPhoto and and thus update to iPhoto 9.6.1, but it requires that you own an AppStore version of iPhoto. A version from the iLife installer DVDs will not suffice. You can neither buy iPhoto any more update it. If you are lucky, you can reinstall and update this way.
Open the App Store and click the fourth tab "Purchased". Is iPhoto listed there? Then move the currently installed, incompatible iPhoto to the Trash and restart the AppStore. Open the Purchased tab and click the download link for iPhoto.
If you cannot find iPhoto in your Purchases, use Photos for Mac instead of iPhoto. Photos for Mac can open your iPhoto Libraries. When you launch Photos the first time, it will open the last iPhoto Library you used and create a Photos Library from it.
See the Help to get started with Photos: https://help.apple.com/photos/mac/1.1/
On a Mac with Yosemite the Photos.app is still very basic and you may want to keep using iPhoto. On the more recent system versions the Photos.app is much better. Could you upgrade the Mac at least to macOS 10.11 El Captan?
