Open these file for edit.
Locate line 387
try {
$connection = new Mysql($this->request->post['db_host'], $this->request->post['db_user'], $this->request->post['db_password'], $this->request->post['db_name'], true);
} catch (AException $e) {
Add below line after try {
require_once DIR_DATABASE . 'mysql.php';