For improved detachability, macOS will unmount any non-internal volumes that are attached to the system when you log out. So, for example, if you log out of your computer while a USB or Thunderbolt hard drive enclosure is attached, you can detach those hard drive enclosures from the system without having to manually unmount them first. This is a good thing — it would be annoying if you had to log back in to your system just to eject a drive. The downside of this, though, is that if you have a CCC backup task that runs when no user is logged in, the destination volume may be unavailable. For a local backup, CCC will attempt to manually mount the destination volume. When the destination of your backup task is a remote Macintosh, however, CCC will not be able to mount that volume prior to backing up.
If you anticipate backing up to a remote Macintosh that may be sitting at the loginwindow, you can change the behavior of macOS to not unmount detachable volumes. To change this behavior, run this command in the Terminal application on the remote Macintosh:
sudo defaults write /Library/Preferences/SystemConfiguration/autodiskmount AutomountDisksWithoutUserLogin -bool YES