Device must be a disk or tape.  On the device field, press PF3.

    Meaning:  You entered a device name other than a disk or tape device.

    Action:   Specify a valid disk or tape device.  Press PF3 to display
              box of known disk devices.  Use the arrow keys to scroll
              through the selection options and press Return to select a
              device.  Selecting a device from this box places the
              selection in the name field.

              If you do not want to select a disk device press PF3 again
              (or Ctrl/Z) to cancel the box.