phpmyadmin cannot login as root user
Note that to use the root user in MySQL / MariaDB you must use sudo. If you remove it, it will give the same error as in PhpMyAdmin. That is, the error comes from MySQL / MariaDB.
:~$ mysql -u root -p ERROR 1698 (28000): Access denied for user 'root'@'localhost'
So, you have to use sudo or to be the system root user.
:~$ sudo mysql -u root -p
Next, create the new user.
>CREATE USER 'new_user'@'%' IDENTIFIED BY 'your-password';
Note: The % sign means that the new user will be able to connect to the MySQL / MariaDB server from any IP.
Then assign the permissions to this new user.
> GRANT ALL PRIVILEGES ON *.* TO 'new_user'@'%' WITH GRANT OPTION;
1.- Fixing the error on the phpmyadmin login
Now open your PHPMyAdmin and you should be able to log in with your new user.