Seeeduino XIAO unable to output serial port signal through UART

[allrobot]

void setup() {
    Serial1.begin(9600);
    SerialUSB.begin(9600);
	    while(!Serial1){}
}

void loop() {
	Serial1.print(analogRead(A0));
    Serial1.flush();
}

The two ports are COM5 and 6, I turned on two port monitors and there was no data output…Why?