It's really a matter of procedure. Hardware Serial ports (D0 and D 1) of arduinoLeonardo are different from those of ArduinoUNO, which correspond to Serial 1 instead of serial. So, you can change the serial number in the program to Serial 1. Arduino officials explain it this way: Notethethontheronardo, serial classrefererstousb (CDC) communication; For serialonpins 0 and 1, use the serial 1 class. Reference: ArduinoBoardLeonardo