What is NSURLErrorDomain error -999

This is typical for problems with with connecting to a server. The error code -999 is given, when another process has cancelled your request. It could be a time out or incorrect or insufficient data when trying to connect to iCloud Photos.

Do you have enough free cloud storage to hold all photos from your Photos Library, in addition to any photos you are already having in iCloud? Do you have enough free storage on your Mac to hold all photos from iCloud Photos that will be downloaded from iCloud?

Hello JohnWellspring,

Thanks for reaching out to Apple Support Communities. We see you have some questions about setting up iCloud Photos on your Mac, and we'd like to help.

Are you following the steps as shown in this article to set up iCloud Photos?

Turn on iCloud Photos

On your Mac

1. Choose Apple menu  > System Preferences.

2. Click Apple ID. 

3. Click iCloud in the sidebar, then select Photos. 

If you have an earlier version of macOS, go to System Preferences > iCloud. Click Options next to Photos, then select iCloud Photos.

Set up and use iCloud Photos - Apple Support

If so, when in the process are you seeing this error? Is it when enabling Photos, or during the upload process?

Here is the Photos User Guide for Mac

Regards.

Great. You’re welcome ☺️