The MKRMotor Carrier is a shield that will work with several different MKR family boards. (You have a Nano-compatible version instead? In that case, it ought to work with several different Nano boars.)
You should set your "board" to "Nano 33 IOT", if that's the board that you're using that contains the CPU.
Never mind; I see you were talking about firmware in the motor board itself.