Get post data, in Checkout success observer

protected $_request;

public function __construct(
    \Magento\Framework\App\RequestInterface $request,
    \Psr\Log\LoggerInterface $logger,
    \Magento\Sales\Model\Order\Status\HistoryFactory $historyFactory,
    \Magento\Sales\Model\OrderFactory $orderFactory
) { 
    $this->_request = $request;
    $this->_logger = $logger;
    $this->_historyFactory = $historyFactory;
    $this->_orderFactory = $orderFactory;
}

Inside function get post data as below way:

$this->_request->getPost();

Don't forget to clear var/generation folder.

I'm pretty sure you need to replace:

$this->getRequest()->getPost()

With:

$observer->getRequest()->getPost()

But it may depend on the event you're observing IIRC.