If you are seeing this error message, it means that your PHP installation is missing the MySQL extension, which is required by WordPress. The MySQL extension is a library that allows PHP to communicate with a MySQL database. Without the MySQL extension, WordPress will not be able to access and use the database, which is an essential part of the application.
To fix this issue, you will need to install the MySQL extension for PHP. The specific steps for doing this will depend on your hosting environment and the version of PHP you are using. You may need to contact your hosting provider or system administrator for assistance with this process. Alternatively, you can try searching online for instructions on how to install the MySQL extension for PHP on your particular setup.