The error message indicates that you are missing the XSL extension in your PHP setup. You can see http://www.php.net/manual/en/xsl.installation.php for more information on installing that extension.
Please note that the XSL extension is not necessary for the default template; you can also choose to use the PHAR executable or the downloadable archive. These can be downloaded from https://github.com/phpDocumentor/phpDocumentor2/releases/latest. These two do not check for the XSL extension and can thus be safely used.