can't scale for external monitor
When opening the Display preference pane in MacBook/MacBook Pro, additional floating window is displayed for the connected external display. The scaling and resolution settings for the external display can be adjusted in that window.

For additional instructions, refer to the Apple Support Document, macOS Sierra: Adjust your display’s resolution.


  Set the resolution for a connected display
  If you have more than one display, additional resolution options are available.
  
  After the display is connected, choose Apple menu > System Preferences, click Displays, then click Display.
  
  Press the Option key while you click Scaled to see additional resolutions for the second display. Then select a scaled resolution option for the second display.