I've seen this question asked on the forums before, but with no help. I hope this gets the attention of someone from the staff.
I created a discount code that should apply to everything, but the field to type in the code is not showing up. I dont have a desktop to test this, but its happening on mobile.
Ok I have identified the problem and solution on my side. I'm using an app called Dynamic Product Options which creates a draft order, and this conflicts with the default checkout process. Fortunately, in their settings they allow you to choose Default Checkout instead of Draft Order. Once I switched back to Default Checkout, the discount code field appeared again as expected.
I hope this helps someone else.