I guess that this is an automatic behavior of the Shopify system.
We had some issues with the vouchers setup and we needed to void them - you should receive new ones very soon.
So likely at this moment we have no discounts available and the system doesn’t even offer the option to input them.
I’m not 100% sure if this is exactly what is happening, but I believe that it’s likely that.
Please wait until you receive the new ones and let us know if the field pops up somewhere or not (an opportunity for us to learn something about Shopify as well 
).
Sorry about the inconvenience.
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