A VDisk cannot simultaneously be both the source of a FlashCopy mapping and the target of a FlashCopy mapping. The target VDisk that you have specified is currently defined as the source of a FlashCopy mapping.
You have two options. One option is specify a different target VDisk and resubmit the command. The other option is delete all of the existing FlashCopy mappings that contain the target VDisk that you have specified and resubmit the command.