Jetson TX2 NX RS232 Communication Not working

Both ends work with loopback. Assuming the cable really is ok (and probably it is), and assuming voltage levels are the same (and you confirmed they are), then the problem has to be different settings at each end, or noise. Noise can still hit via a ground loop even if the units are otherwise 100% correct. Settings are most likely differing between the two. Beware that as speed goes up past 115200 (the default speed) the clock tolerance will not be quite correct at the Jetson end (so if you are trying speeds in the half Mbit/s or up range you will need to mention this), and this is one form of mismatched settings. I’m probably overlooking it, but I don’t see the exact settings listed above. If the speed is not high enough, then I wonder about a ground loop causing noise.