Photos cannot find the System Photo Library named “Photos Library.photoslibrary”. I recently migrated from an old iMac to a brand new one and get this message. No access at all to photos and no folder either. Ideas?

You need to locate where your photo library is stored. Usually it's in your Pictures folder in your Home folder (Macintosh HD > Users > (your username) > Pictures)

If it's not there, you might need to search for it. In either Spotlight or the Finder search box, type in ".photoslibrary" in the search box. Note the leading period and the "photos" is plural and there is no space between "photos" and "library". This will normally find all photo libraries your computer.

If you used Spotlight, click the "Show all in Finder" to see a list with details.

In the Finder results, clicking once on each will show you in the status bar at the bottom where the file is stored. Just double click one to open it. Once you do this, it will become the default library because Photos always tries to open the last library you used. Note that this is different from a "System Photo Library" which is especially important if you use iCloud Photos. But don't worry about that yet. Just make sure the one you open has all your photos. If it doesn't, try the others in the list until you find the right one. And you might want to make a mental note of where that was.

Hope this helps.

That tells me that your Picture folder may have been moved or didn’t make it over from your old iMac. You may need to try copying it over from the old iMac again.