If the markets have closed and you’ve place an order, it remains pending until the markets reopen. You can review and request to cancel your pending orders in the Transactions section of Hatch (click on the 'Pending' tab), however, there are times when you can't cancel an order.

Market orders can't be cancelled while the markets are open, but limit and stop orders can be. No orders can be cancelled within the two minutes before the markets open.

If the markets are open, chances are your order would be placed immediately. You can't reverse a processed order, so if an order has been processed, you'll need to wait for it to settle first, then buy/sell it. 
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​Note: The request to cancel an order does not guarantee that the order will be cancelled by our broker before the order is processed.