The MySQL extension is missing. Please check your PHP configuration. See our documentation for more information.

The error message you provided indicates that the PHP MySQLi extension is missing or not enabled in your PHP configuration. The MySQLi extension requires PHP to interact with MySQL databases using the improved MySQLi functions.

Open the terminal and install it.

At first, Check your php version 

php -version

sudo apt-get install php7.3-mysqli

And last restart your apache2 

sudo systemctl restart apache2