You are using an outdated version of the avr-debugger library, please upgrade it to
lib_deps = jdolinay/avr-debugger@^1.5
per library page.
Yes, exactly – we’ve already talked about this in Linker error after adding the RS485 library - #2 by maxgerhardt, since the avr-debugger library is default-configured to use the Serial hardware (USART0), all usages of Serial in your firmware code are forbidden, attempting to use it will cause a linker error. As is documented.
Serial uses USART0.
Serial1 uses USART1 (Rx=PD2=D19, Tx=PD3=D18), etc.