error downloading a high quality version of photos

This error message may occur, if you are using a Photos Library, that is syncing with iCloud Photos and want to use a photo for adding adjustments or sharing the photo. Photos may need to download the full size original from iCloud or an adjusted version of the photo.

The download can fail for several reasons:

1. If Apple's cloud servers cannot be reached because the server is offline or because a firewall or other third-party security software is blocking the access to the cloud servers. Do you have any firewall or antivirus software installed, for example Little Snitch or Kaperski? You can check the iCloud status here: Apple - Support - System Status
2. if the photo is missing in iCloud because you previously deleted it from iCloud, while your current library has not been syncing with iCloud.
3. If you have not enough free storage on your Mac or in iCloud - have you checked your storage?
4. If the photo has been edited with third-party software, but does not have a high resolution preview - this can happen, if the pho has been edited with Aperture or iPhoto - then you should revert it to the original before trying to add further adjustments.