When the Arduino is connected via the USB serial port it should be “visible” to the Jetson Nano. You should be able to verify that with command “lsusb”, this should give you a listing of devices on the USB bus. See also this video (for Jetson TX1 but should be applicable for the Nano as well):
You probably need to disable the nvgetty service first as mentioned in this topic: Jetson Nano how to use UART on /ttyTHS1 - Jetson Nano - NVIDIA Developer Forums