Does it work there? (You can open the serial monitor standalone without needing to upload a program).
The only major difference between PlatformIO’s serial monitor (aka, miniterm.py) and Arduino’s serial monitor is that directly sends data on every keypress, not on enter. But you can make it behave the same by using a monitor_filters = send_on_enter directive in the platformio.ini.
Otherwise as a general sanity check, check the validity of
I.e., that COM9 is the right serial port (otherwise set monitor_port), that 9600 is the baud rate used in the firmware in Serial.begin() (otherwise set monitor_speed) and that the USB serial adapter is physically connected to the right pins on the target microcontroller (GND, TX, RX).