The HX711 is a digital device. You don't read the output of the HX711 using the analog inputs, so remove the connection to A0.
It looks like you have mixed in some PID code intended for a completely different type of sensor.
Make sure that the HX711 and load cell is working correctly, and that you understand the output, before trying anything else. Sparkfun has a clear guide describing how to connect the HX711 to Arduino.
If you want to use the values produced by the HX711 as input for a PID controller, that is fine. However, those input values would be contained within a variable internal to your program.