Photos Cannot Find The System Photo Library, Fix

With Photos closed, when you option click on Photos it presents a dialog asking you to choose the library you want to open, and it indicates which one is your system library. If it doesn't show, you should try pointing it to your Pictures folder. Or you can, with the same dialog, create a new library to put new pictures into. In Photos>Preferences you can set your current library to the system library, if you want.

Or-- you can go to your Pictures folder and double click on the library you want, maybe "Photos Library.photoslibrary," and it will open in Photos.