The mysqli|mysql extension is missing.

In this tutorial is The mbstring extension is missing. Please check your PHP configuration 18.04.

Just install php-mysql. mysql extension is missing is main your PHP version and MySQL is not comfortable. First, open your command prompt and check PHP version with bellow command:

$ php -v

Please check your PHP version with MySQL comfortably.

sudo apt-get install php7.2-mysql

Almost done now restart your apache with:

sudo service apache2 restart

Now open again phpmyadmin like: 127.0.0.1/phpmyadmin