Q: iPhoto vs photos what's the difference

Photos is the relatively new photo managing app. iPhoto was the app that came with iLife that was discontinued, though it still works fine. Where are you seeing this error?

Main difference is that iPhoto is dead all support having been discontinues nearly two years ago and print products having been discontinued about six months ago  and Photos is the current Photos manage software included with all Macs and is fully supported

Photos is  a new program and has a learning curve and required new work flows - this may help some Photos 1.5 vs iPhoto 9.6.1:  Features and Capabilities and this How Photos handles content and metadata from iPhoto and Aperture - Apple Support

Have you migrated your iPhoto library to Photos ?

LN