The VDisk is currently online. The migration process assigns the VDisk an online or offline state based on the states of the source and target MDisks. In this case, based on the offline state of the target MDisk, the VDisk would have been taken offline. The task cannot be initiated because this action is not supported.
Bring the target MDisk online by following the recommended procedure for bringing an MDisk online, and resubmit the command.