In DigitalOcean, running the do-release-upgrade command was returning the following message.
Checking for a new Ubuntu release
Please install all available updates for your release before upgrading.
sudo apt-get update
sudo apt-get upgrade -y
sudo apt-get dist-upgrade
shutdown -r now
You could stop here if all you want is to install available updates. Read the warning below to make sure you don't break your live applications and whether this is the best approach you can take.
WARNING: Please read this article by DigitalOcean on the potential pitfalls of upgrading an existing installation with your applications running on it. Instead of upgrading in-place, the recommended approach is to migrate your applications by creating a new, fresh instance with Ubuntu 20.04 LTS instead of upgrading an existing one. (Run at your own risk!)
sudo do-release-upgrade