[macOS Photos] How to fix “Unable to upload to iCloud”

  1. Open Photos
  2. Open Preferences
  3. Uncheck “My Photo Stream” if it is displayed.
  4. Uncheck “iCloud Photos” – This will delete thumbnails on your mac.
  5. Wait until all the thumbnails are deleted.
  6. Check “iCloud Photos” again. This will take some time. But this time, you won’t see any upload error.