I’m familiarized with bash, so I think I could give a try…
One thing I wonder…what does the command say (I’m looking for a difference) if no Jetson is connected to USB at all when you run the command? Then connect the Jetson in recovery mode, make sure “lsusb -d 0955:7c18” finds the Jetson, and run the command again. Is there any difference in output?
If you find any particular line in flash.sh which triggers the “not a valid target”, post that here. If you try the command again, with nothing connected, and using targets “jetson-tx1” and possibly “jetson-tk1” or “jetson-xavier”, see if that also fails the same way (depending on release those targets might or might not be valid).
Does the issue repeat if you use different computer with Ubuntu or different Jetpack version with different computer?