Upgrading from Ubuntu 19.04 (EOL) to Ubuntu 20.04

Solution 1:

I think the problem is simply that you are trying to skip past intermediate versions, which the do-release-upgrade tooling does not support.

Non-LTS versions upgrade to the next version.

LTS versions upgrade to the next LTS version (unless specifically configured to upgrade to a non-LTS version).

As 19.04 is a non-LTS version the immediate upgrade step is 19.10, then the next step would be 20.04.

The configuration for what type of version to upgrade to is in /etc/update-manager/release-upgrades, but keep in mind that it essentially decides where to go next when you currently are at an LTS version, there are really no alternatives when at a non-LTS version.