Error when exporting iPhone Photos

1. The error occurs when you accidentally update the Mac while backing up the photos. Try to disconnect the external device and restart the export again.
2. If you experience problems with photos in a library, you can try to repair it. 

Important: Make sure to back up your photo library before repairing it. 

1. If the Photos app  on your Mac is open, choose Photos > Quit Photos.
2. Press Option-Command and double-click the Photos icon in the Applications folder (or Option-Command click the Photos icon in the Dock).
3. In the Repair Library window, click Repair to rebuild your photo library.