Can't upgrade Ubuntu 16.04 to newer release

Solution 1:

In short, you waited too long to do this easily and effectively.

16.04 is already past standard support, which means that there are no longer updates unless you are on Extended Security Maintenance (ESM). You need to perform updates before you can upgrade to the next release, which you can't do without using ESM. Hence the problem.

You could sign up for ESM, perform your system maintenance, and theoretically should be able to upgrade to 18.04 without incident.

But since you want to be on 20.04, it doesn't really make any sense to release upgrade. You can't easily skip over releases when upgrading and there's always a potential for an upgrade to be unsuccessful, depending on your hardware and whatever is already installed on your system, especially if you have any software that didn't come from Ubuntu repos. Release upgrades also take a long time to complete.

It's probably better that you cleanly install 20.04 at this point. Remember that LTS releases come with 5 years of standard support and if you don't want to be in this predicament in the future, make sure you upgrade in a timely manner. You have three whole years to upgrade after a new LTS to the next LTS before it becomes unsupported.

You can also sign up for the ubuntu-announce mailing list to get emails about new releases and EOL announcements:

https://lists.ubuntu.com/mailman/listinfo/ubuntu-announce