System Photo Library

Is the Photos Library on your Mac your System Photo Library?

• Open Photos on your Mac, if it is not running.
• Open the Photos > Preferences > General and check the item "Use as System Photos Library". If this is not dimmed, click it, to make the library your System Photos Library.
• Check the line "Library Location" in the Photos Preferences. This line is showing you the location of your current Photos Library. Is the library in a supported location, for example the Pictures folder? If your library is on an external volume, the volume needs to be locally mounted, with a USB or thunderbolt connection, not on a NAS or a in a synced folder, like Google Drive or Dropbox. The external volume must not have been used for Time Machine backups.
• Then restart Photos.
• Try to sync again.