DL tends to insert "can" into mild commands such as "me das la leche" because it's not considered impolite in spanish. I cringe though because "will you give me the milk" is far better english than "can you give me the milk." However in a situation like this one, "cannot" clearly denotes capacity to do so rather than idiomatic phrasing. They've got to fix this.
I "get" what Duo is trying to teach me here.
BUT... In a conversation, there might be some context which would clue me in either to use or not to use "can". There is NO context here.
Who knows whether this a statement of fact or a statement of frustration or a statement of inability to accomplish a task???
The lack of context makes this a guessing game!