There was a thread very similar to this one a few days ago.
https://forum.arduino.cc/t/digital-scale-rs232-max3232-with-arduino-mega/1259275
The reason that the scale would not communicate with the Arduino, was that the GND pin inside the scale was connected to a different pin to that which was shown on the schematic.
It might pay you to look inside your scale, and check the connector wiring.