Your PHP installation appears to be missing the MySQL extension which is required by WordPress

Run the following command to fix the Your PHP installation appears to be missing the MySQL extension which is required by WordPress issue:

sudo apt update 
sudo apt install php-mysql 

You will get the output like this:

Unpacking php7.4-mysql (7.4.3-4ubuntu2.10) ...
Selecting previously unselected package php-mysql.
Preparing to unpack .../php-mysql_2%3a7.4+75_all.deb ...
Unpacking php-mysql (2:7.4+75) ...
Setting up php7.4-mysql (7.4.3-4ubuntu2.10) ...
Creating config file /etc/php/7.4/mods-available/mysqlnd.ini with new version
Creating config file /etc/php/7.4/mods-available/mysqli.ini with new version
Creating config file /etc/php/7.4/mods-available/pdo_mysql.ini with new version
Setting up php-mysql (2:7.4+75) ...
Processing triggers for php7.4-fpm (7.4.3-4ubuntu2.10) ...
Processing triggers for php7.4-cli (7.4.3-4ubuntu2.10) ...

Restart the php and nginx service after that.

sudo systemctl restart nginx
sudo systemctl restart php7.4-fpm

Your issue will be fixed now.