If you can't see the field to input the discount code, you need to go to your Admin, click on Apps and select Discounted Pricing App. You need to go into the settings to ensure it is 'enabled'.
Enable discount view on Discounted Pricing app
Ensure that the discount code field is shown along with the already discounted cart by changing your Settings in Discounted Pricing app.
Under "General Settings" on the app, you can find the slider to "Allow customers to use a discount code in conjunction with their discounted cart."
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