Hi minkyu kim,
For normal mode, after the slave board send message, then master board will receive RX_MSG_ID(2UL) and toggle LEDs.

But you are using loop back mode, the message ID that you send was "TX_MSG_ID (1UL)", it did not match the "RX_MSG_ID (2UL)".
You can try to modify the value of them. When the value are same, the LEDs will work in loop back mode.
For example, modify:
RX_MSG_ID (1UL)
Then after press SW2 and SW3, the LED will show RED or GREEN.
Best Regards,
Robin
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