JETPACK 4.X = UBUNTU 18.04 || JETPACK 5.X = UBUNTU 20.04
JETPACK 4.X only received bug and security updates(Ubuntu 20.04 on Nano? - #6 by hpc88323)
If you want to use Ubuntu 20.04 you will have to buy a newer Jetson compatible with JETPACK 5.X(Jetson Software Roadmap for 2H-2021 and 2022)
