Serial is not on /dev/ttyS0 as far as I have seen.
You are going to have to tell us how you have connected the serial. For example, the R-Pi has hardware serial (/dev/ttyAMA0) but you have to enable it with the raspi-config tool to use it. Some use a USB serial bridge (/dev/ttyUSB0), and some connect to an Arduino Uno (/dev/ttyACM0).