A volume cannot simultaneously be both the source of a FlashCopy mapping and the target of a FlashCopy mapping. The source volume 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 volume and resubmit the command. The other option is delete the existing FlashCopy mapping that defines the source volume that you have specified as the target volume, and resubmit the command.