As decsribed in Metalab site "AVR's UART lines are not used for USB. This means that they can be used for other purposes, but also that debug output written to the UART cannot be directly received via USB."
As decsribed in Metalab site "AVR's UART lines are not used for USB. This means that they can be used for other purposes, but also that debug output written to the UART cannot be directly received via USB."