I just signed up for Lightroom CC, and I've been working to sync Lightroom Classic (7.1, Windows 10) with the cloud. I probably had to kill the program and restart it 50 times for it to get most of the way through syncing my 50,000 photo collection. Finally, we are down to about 40 photos, and it won't budge.
I checked the list of files to be synced in Preferences, and the first two files on the list seem to be failed uploads from Lightroom for iPhone--from a CC trial i did two years ago. Once I finally figured out how to tell what the sync error was, it tells me to open Lightroom on the phone and allow it to finish uploading. I don't have the phone anymore and deleted Lightroom from it years ago. How can I get delete these missing "images" out of my cloud database so it will stop trying to sync them to Lightroom Classic?
Log into your Creative Cloud account with a browser by going to adobe.com. Make sure you are signed in to your account. After you are signed in you will see a dark grey box on the left of the screen with links to all kinds of things, including Lightroom. Look for the Lr icon. Click on the Lr icon to go to your synced Lr content. Click on the All photos option and look for images without previews. These are the files that never uploaded. You can select and delete them from here. That will clear out the sync queue back in Lr on your desktop.