Having a gift card refused at checkout is frustrating, and it is usually something small. "Enter a valid discount code or gift card" only means the checkout did not recognise the code as entered.
Use the code from the gift card email itself. The order number on your purchase confirmation is not the gift card code.
Check the right inbox. The digital gift card is emailed to the recipient address entered on the gift card page when it was bought, so if you bought one for yourself, look at that address, including its spam or promotions folder. Can I purchase a gift card for someone else? covers how the card is sent.
Copy and paste the code rather than retyping it, and make sure no space has been picked up before or after it.
The same message also appears for promotion codes. If what you are entering is a promotional code rather than a gift card, see Why is my promo code not working?, as the causes are different.
If the code is still refused, message us here, or email [email protected], with the order number of the gift card purchase and the email address it was sent to, and we will look into it.