Arduino Mega serial output not working

I have a board with 2 processers. One of Arduino Uno and another one of Arduino Mega. They are on one I2C bus. I can successfully upload my codes to all the processors separately and they work with no issue. I can get the serial print information from the Arduino Uno processor with a serial debug cable, and I2C works good too. But the issue is I cannot get serial print information from the Arduino Mega processor. I have tried to change the version of both c/c++ and platformIO extension, I am using vs code. Because I am assuming it is caused by version, but still does not work. Anyone had this issue before?

Please show your exact schematic, platformio.ini and all (hopefully minimal) code involved.