PhpMyAdmin, Access denied for user 'root'@'localhost'

Question

I'm using ubuntu 2018, I have just installed phpMyAdmin but I get this message:

#1698 - Access denied for user 'root'@'localhost'

I have followed this steps: Cannot enter phpmyadmin as root (MySQL 5.7)

but I get other error in the terminal:

mysql> CREATE USER 'phpmyadmin'@'localhost' IDENTIFIED BY 'mysql';
ERROR 1396 (HY000): Operation CREATE USER failed for 'phpmyadmin'@'localhost'

EDIT I have found a solution in the documentation:

sudo mysql

Then

GRANT ALL ON *.* TO 'nom_utilisateur_choisi'@'localhost' IDENTIFIED BY 'mot_de_passe_solide' WITH GRANT OPTION;
FLUSH PRIVILEGES;
QUIT;

But when I have copied and paste the last command , it has directly created user with name "nom_utilisateur_choisi" and password "mot_de_passe_solide".

How can I edit or delete them to repeat again ?