Had the same issue as u mate, however i found a nice article where the upgrade possbilities were described and if you really like to do an upgrade on your arm board, give it a try with the debian way described here:
https://linuxconfig.org/how-to-upgrade-ubuntu-to-20-04-lts-focal-fossalts
If you still run then into problems as i had with apt update i did following:
sudo apt purge command-not-found
sudo apt --purge autoremove
sudo bash -c 'apt update && apt full-upgrade && apt autoremove && apt clean'
No running Ubuntu Focal with Armbian Linux and everything else seems to work so far for me!
