If you get an error indicating "Your PHP installation appears to be missing the MySQL extension which is required by WordPress", it means that you are using a PHP version that is incompatible with your script's version.
This usually happens if your WordPress installation is outdated, as the MySQL extension is deprecated (no longer supported) from PHP 7.0 and beyond. To fix this, it is strongly recommended that you update your WordPress installation, themes, and plugins to their latest versions, both for security and performance reasons.
As a temporary solution, you can change the PHP version of your website to 5.6 or lower by following this guide: How to Change the PHP Version for Subfolders.