Follow instructions in:
https://github.com/Qengineering/Jetson-Nano-Ubuntu-20-image?tab=readme-ov-file
Format Micro SD Card with 256GB using SDCard Fomatter:
With balena etcher (in Administration mode) flash the image as downloaded into a 256GB MicroSD Card. Do not to unzip, just flash JetsonNanoUb20_baer.img.xz.
The flash for the other images I could not made them boot the Jetson
Apendix:
In case need to upgrade from Ubuntu 18.04 to Ubuntu 20.04 follow:
https://qengineering.eu/install-ubuntu-20.04-on-jetson-nano.html