The wizard concatenates the target name to the end of the source VDisk name and then concatenates a sequence number at the end of that string to create the new VDisk names. Because you have chosen a source VDisk that has a default name format and you have not specified a target name, the concatenation would result in VDisk names that also have the default name format, which is not supported for this wizard.
Either select a VDisk with a name that is not in the default format 'vdiskn' where n is numeric or specify a target name when you submit this wizard.