No, once an order has been placed (meaning payment has been taken and you have received an order reference number or confirmation email), it is too late to apply a discount code. This applies even if your order is has not yet shipped.

You must ensure that your discount code is applied before completing your order.

• For one-time purchases, the code must be applied at checkout. See: How do I apply a discount code to a new order?
  
  
• For subscriptions, discount codes can be applied in the subscription portal at any time, but they will only apply to future scheduled orders. They cannot be applied to any orders that have already been processed. See: How to add a discount code to your subscription