You’re back in iPhoto, if you have a photo library it will show up, if you don’t, you’ll be on a fresh iPhoto screen like so: Return to the iPhoto app in /Applications/ and launch it as normal, you may want to drop the icon into the OS X Dock for easy future access.Click on “Install” next to iPhoto, this will download and install the newest version which is compatible with OS X 10.10.3+.
Most commonly, Mac users will encounter the following error message when trying to launch iPhoto on a Mac with Photos app: “In order to open “iPhoto.app”, you must update to the latest version.