IPhoto

That is a generic message. Photos replaced iPhoto several years ago. To access your media: Launch Photos with the option key held down. In the resulting dialog, choose your iPhoto Library. Photos will create a new library based on your iPhotos media. Your iPhoto Library will not be changed.

iPhoto was discontinued more than 4 years ago. It is no longer developed or sold. It was replaced by the more powerful (and free) Photos.app that comes with the OS. You can migrate to that app very simply:

Hold down the option (or alt) key and launch Photos. From the resulting menu select 'Choose Library' and point at your iPhoto Library. It will do the rest.

If you prefer not to use Photos there are several 3rd party apps that you can use. Which to choose depends on the kind of photographer you are and your budget.