Testing UART connection

I am testing the UART connection on my PLT-300A by attempting to turn on an LED on my PCB using UART communication. The code below uses "S0\n" as the intended UART command to activate the LED. Although the code passes successfully, none of the LEDs light up when it runs. The LEDs I am trying to control are on DDTP08, DDTP09, and DDTP010. Could you please advise if there are any configurations that need to be set beforehand or if these details need to be included in the code?

title: UART test