Have you upgraded to Yosemite and having issues upgrading iPhoto?
You’re not alone. It also appears that no one knows what is causing the issue. Consensus seems to be that deleting iPhoto and re-installing will resolve the problem.
Here are the steps:
Go to the applications folder locate the iPhoto application.
Delete the application (move it to the trash) -> empty the trash.
You shouldn’t loose your photos, they are stored in your userfolder/pictures/iPhoto Library. If you’re concerned about it, you can copy that file to an external drive.
Restart your mac (probably not necessary, but just in case)
Open the App Store
Search for iPhoto and download the app.
iPhoto is a free app, so you shouldn’t have to pay for it.