If you are seeing Your PHP installation appears to be missing the MySQL extension which is required by WordPress error it means that you are using a PHP version that is incompatible with your script's version (your WordPress installation is most likely outdated).
Since the MySQL extension is deprecated (no longer supported) from PHP 7.0 and beyond, changing your PHP version to 5.6 or lower will fix the issue. It can be done as guided here: How can I change my PHP version?
Our suggestion would be to update WordPress altogether, so such issue wouldn't occur in the first place.