Seeeduino V3.0 (Atmega 328P) and Rotary Angle Sensor Mystery Message
One problem is that you try to generate serial output before you initiate Serial.  This could have odd results beyond just making that message fail.

Serial.println("Setup Start...");
Serial.begin(9600);


Should be

Serial.begin(9600);
Serial.println("Setup Start...");


And of course your posted code never reads the sensor...