Just an additional hint: in the mapping toolbox, if something is already set in the ecommerce extension, it usually says "(override _cxxx)" where _cxxx is one of the e-commerce variables e.g. "Product Discount (override _cpdisc)". Anything that does not say "override" usually means you must manually map this value if you wish to send it.
This is not always the case, but it's one way of checking. In this case, the wording tells you that DCNT is coming from the e-commerce extension, and you have the option to override it if you wish, but "DISCOUNT", as Josh said, must be manually mapped ("Map a value to DISCOUNT" infers that the tag does not do it automatically for you).
Check out our new Swift integration library for iOS, macOS, tvOS and watchOS: https://github.com/Tealium/tealium-swift with updated