Recently Apple enabled a default option to sync the Desktop and Documents folders with iCloud. This may result in an error if you attempt to upload files from your Desktop or Documents folder that are already uploaded to iCloud and not physically on your computer.
Disable iCloud synchronization for your Desktop and Documents folders
1. To disable iCloud synchronization for your Desktop and Documents folders, first make sure the files on your Desktop and Documents folders are downloaded to your computer from iCloud, and move them to a different folder that isn't being synced, such as the Downloads folder.
2. From your Mac, click on the Apple menu and select System Settings or System Preferences.
3. Click your Apple ID, and select iCloud.
4. In the Apps Using iCloud section, click iCloud Drive.
5. Make sure that Sync this Mac is toggled On, and click to toggle Off the Desktop & Documents Folders. Then click Done.
6. Then move the files back to your Desktop and Documents folders and they will no longer be synced with iCloud automatically.