updating firmware atmega16u2 on UNO R3

  1. select atmega 16u2.
  2. communicate with USB.
  3. load a .hex file(arduino-keyboard.hex)
  4. erase and verify device.
  5. press start application.
  6. And it said "Could not write to USB device." I want to make USB keyboard. How can I solve this problem? (Sorry for my English. :sweat_smile:)