This issue comes from some reason. But last time when i tried to setup ubuntu i see the problem happens frequently. So i can say most of the case from new linux OS install. Giving you two quick shot method.
Solution 1 :For PHPMyAdmin create a new Superuser
you can run mysql from your terminal with sudo like this below command.
sudo mysql -p -u root
Now add a new user with username you want to set. just chaneg the ‘dbuser’ and ‘dbpassword’ as your own choice.
CREATE USER 'dbuser'@'localhost' IDENTIFIED WITH mysql_native_password BY 'dbpassword';
Let grant this user as a super user/super privilege’s
GRANT ALL PRIVILEGES ON *.* TO 'pmauser'@'localhost' WITH GRANT OPTION;
Hope you can login with new username & password you set.
Solution 2 : Authentication method changing for root
For login into phpmyadmin you have to switchauth_sockettomysql_native_password.
Open the terminal and run this commands below.
sudo mysql -p -u root
by run this command you can see your current plugin(auth_socket).
SELECT user,plugin,host FROM mysql.user WHERE user = 'root';
Now change this plugin with a new password. make sure you will change the password ‘dbpassword’ according to your own passwor.
ALTER USER 'root'@'localhost' IDENTIFIED WITH mysql_native_password BY 'enter_password_here';
nice ! lets flush privilege
FLUSH PRIVILEGES;
Now exit the mysql terminal and try login. Hope it will works. If please restart mysql server. Besides you can comment if there any issues still you are facing.