hello Henry.Lou,
could you please check Jetson TX2 Series OEM Product Design Guide for the [13.3 UART] chapter. please review your physical connections with [Figure 46. UART Connections].
besides, UART7/UARTG is used for BPMP debugging, you have to disable BPMP console on UART7 if you need to enable that. please check Topic 54572 for changing the port number to 255.