Try this:

sudo dpkg-reconfigure phpmyadmin

I had the same problem. Try looking in /etc/phpmyadmin/config-db.php
I found that the default dbuser is phpmyadmin instead of root

phpMyAdmin is included in most Linux distributions. It is recommended to use distribution packages when possible - they usually provide integration to your distribution and you will automatically get security updates from your distribution.

I still haven’t figured it out. Any ideas how to get into phpmydmin?