phpmyadmin has no installation candidate

Currently there's no phpmyadmin package in the Ubuntu 19.10 repositories. As you may know, Ubuntu is based on Debian. The person who used to build Debian packages of phpMyAdmin doesn't have the time for it anymore, so there are no current phpmyadmin packages for Debian. And without current packages for Debian, (at least in this case) there are no current packages for Ubuntu.

Some people are working on phpmyadmin packages for Debian, and once those are finished, they can be adapted for Ubuntu 19.10.

So you can either wait until the volunteers for Debian and Ubuntu have created and tested those packages, or install phpMyAdmin with some other method. It's basically a PHP app, so you can just put it into a folder on a server where PHP gets executed. Or you use Composer, or...

wget https://files.phpmyadmin.net/phpMyAdmin/4.9.1/phpMyAdmin-4.9.1-english.tar.gz
tar xvzf phpMyAdmin-4.9.1-english.tar.gz
sudo cp -avr ./phpMyAdmin-4.9.1-english/. /usr/share/phpmyadmin

I would use 18.04 and LTS Enablement for Mariadb and phpMyAdmin. It is 5 year support LTS