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.