An upgrade from ‘disco’ (19.04) to ‘focal’ (20.04) is not supported with this tool.

Unfortunately once a version of Ubuntu becomes unsupported you can run into problems upgrading to the latest version. As is the case when you try to upgrade disco to focal. Ubunut 19.04 to 20.04.

A work around is to update the apt sources and then run an update

Update Apt Sources with.

sudo sed -i 's/disco/focal/g' /etc/apt/sources.list

Now Upgrade with the following two commands.

sudo apt update
sudo apt -y dist-upgrade

More info at the following link.