You're asking it to partition a whole disk - it wipes existing volumes, and creates the ones you specify.
But, you're giving it /dev/disk1s2, which is itself a partition.
If you're trying to reformat the whole drive, you should be using /dev/disk1
Also, typically, people use eraseDisk to wipe and reformat it -
diskutil eraseDisk Usage: diskutil eraseDisk format name
MountPoint|DiskIdentifier|DeviceNode (Re)-partition a whole disk (create a new partition map). This completely erases any existing
data on the given whole disk; all volumes on this disk will be
destroyed. Format is the specific file system name you want to erase
it as (HFS+, etc.). Name is the (new) volume name (subject to file
system naming restrictions), or can be specified as %noformat% to skip
initialization (newfs). You cannot erase the boot disk. Ownership of
the affected disk is required. Example: diskutil eraseDisk JHFS+