This error appears when trying to add user's home folder to Sync.
It only appears in Sync Home Pro or Sync Business, i.e. in licensed Sync version. The thing is that user's home folder contains Sync's storage folder with "License" directory inside. Since license is automatically shared across all linked devices, and via seats in Business version, it's giving the error message.
Generally syncing the whole home folder may no be flawless as it also contains other system settings, other apps' configuration files, etc that are not available for Sync or change too frequently.
Best solution here is to add wanted folders to Sync separately.