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