This is Elias from Shopify Support. I'd be happy to lend a hand with this!
I understand you're able to create a discount code; however, the discount field isn't appearing in the checkout. it's important to note that the discount box will only appear in the checkout when a store has active discounts running. If you haven't created a discount code or if you don't have any discounts active, then the discount box will be automatically hidden from the checkout.
So that I can take a closer look, can you confirm if you've gone through with creating a discount code for your store?