Lightroom Classic: What does "open lightroom on unknown to continue syncing" mean? I can't find the anwer anywhere!

Normally that message means that you have a partial sync that needs to finish. A device (phone or tablet), the Lightroom for Web interface, or Lightroom Classic Desktop (Smart Preview) have started syncing a photo and were closed before they finished syncing the photo to the cloud.  The cloud has a partial sync for a particular photo and cannot sync it down to your Lightroom CC desktop. 

The normal cure is to open any device/browser/Lightroom Classic that is syncing to your Adobe ID and allow it to finish syncing.  Once syncing is complete you can then sync the file down to Lightroom CC and the message should no longer appear. 

OMG!  I thought they were all sync'd months ago and deleted the originals.  I have been paying $10 a month for nothing!  My treasured pics are history!  Argggghhhhh!

Where were these original files Vicki? Like, what kind of device?  You understand that Lightroom Classic never syncs originals to the cloud, so you'd need to keep local copies of those? The Lightroom CC apps, such as the mobile ones, do send originals to the cloud if you've let them finish syncing.