Disable the discount box on Shopify Mobile Web Checkout

I've created a Shopify store where discount codes can be entered indirectly. Therefore I am trying to disable the discount box as seen below.

I've researched it and seen you can edit the code in Themes > Actions > Edit Code. And I've followed some videos: https://www.youtube.com/watch?v=EJi6zfc63RU

However I am unable to do it.

Has anyone done this before?

Please Help!