Help, I am getting a very similar issue to this.
Let me lay out the facts:
Have the new 'Photos' app on my Mac and the old 'iPhotos' app won't open now as it says it needs upgrading and it is no longer on the app store.
My iPhotos library was working fine previous to upgrading.
In my pictures directory is 'iPhotos Library' which is a 21Mb file (with all my photos)
When I click on 'Photos' to open it says 'Photos cannot find the system photo library named "PhotosLibrary.photolibary" with the option to Quid or Open another.
On selecting Open Another there is no libraries in the list so I could create new or click on other libary