You are inside a namespace so you should use \Exception to specify the global namespace:
try {
$this->buildXMLHeader();
} catch (\Exception $e) {
return $e->getMessage();
}
In your code you've used catch (Exception $e) so Exception is being searched in/as:
App\Services\PayUService\Exception
Since there is no Exception class inside App\Services\PayUService so it's not being triggered. Alternatively, you can use a use statement at the top of your class like use Exception; and then you can use catch (Exception $e).