There are two ways to add a discount code, depending on whether you've signed into 'SHOP PAY' or not.
OPTION 1: On desktop devices you'll find the discount field on the right:
OPTION 2: If you've signed into 'SHOP PAY' you'll need to scroll down to find the discount field in the Order summary section (after Enter card section):
OPTION 1: After adding your address and selecting the Shipping method you'll be taken to this payment screen.
You'll find the discount field before the payment section:
OPTION 2: If you sign into Shop Pay when entering your address, you'll be taken to the 'SHOP PAY' checkout. Here you'll find the discount field by scrolling down to the Order summery section (after Enter card section):