We would suggest verify with the latest release first. Currently, it is JP5.1.1(R35.3.1) for Xavier NX.
It seems the size issue during flashing image to NVMe.
You should check the physical size of your SSD, the size in XML and determine the size for rootfs through -S parameter in flash command.
You could refer to the following thread for this:
How to solve the issue that ssd are not entiely availiable after full disk encryption - #5 by KevinFFF