The number of FlashCopy mappings in which a VDisk can be defined as the source VDisk is limited. The source VDisk that you have specified cannot be defined to another FlashCopy mapping because it is already defined as the source VDisk to the maximum number of FlashCopy mappings.
You have two options. One option is specify a different source VDisk and resubmit the command. The other option is delete one of the existing FlashCopy mappings that contains the source VDisk and resubmit the command.