MySQL 5.7 changed the secure model: now MySQL root login requires a sudo
The simplest (and safest) solution will be create a new user and grant required privileges.
1. Connect to mysql
sudo mysql --user=root mysql
2. Create a user for phpMyAdmin
CREATE USER 'phpmyadmin'@'localhost' IDENTIFIED BY 'some_pass';
GRANT ALL PRIVILEGES ON *.* TO 'phpmyadmin'@'localhost' WITH GRANT OPTION;
FLUSH PRIVILEGES;
Reference - https://askubuntu.com/questions/763336/cannot-enter-phpmyadmin-as-root-mysql-5-7