As much as I want to test it out and mess around with armflang Fortran compiler, looks like the JetPack version for Jetson Nano Developer Kit (regular 4GB version) will be “locked” to Ubuntu 16.04 LTS? At the time of writing the download links for the deb packages from Arm are only for 18.04 LTS and 20.04 LTS…
For several releases now the Nano’s release is Ubuntu 18.04. It was back in the TK1 days that it was 16.04. I’d be very surprised if your Nano actually has 16.04. What do you see from “cat /etc/issue”?