How can you tell the difference between 'Let's say that we have not done it' and 'We say that we have not done it'?
Everything in this section sounds like something either a mother, a girlfriend, or a collection of children would say. This falls in the later.
This is definitely correct English too. And means just the same as 'we haven't done it' - that a task hasn't been completed. But in some contexts 'don't have it done' implies a slightly more impersonal aspect, as though you could have used additional people outside of the group 'we'. So it could be used almost interchangeably, but you'll see/hear it less often than Duo's translation
I am not a natural English speaker but this sounds unnatural to me.
Usually in the perfect tenses in English, they use the verb have as an auxiliar. When you make a negative statement, you usually use the auxiliar and the negation word (not, never, nothing, nobody,etc).
In Italian, the passato prossimo tense is very similar to the English's present perfect. Both use the verb *"have/avere" and the participle of the target verb.
I have eaten - (io) ho mangiato.
I have not (or haven't) eaten - (io) non ho mangiato.
Also, the passato prossimo sometimes can be translated as the simple past in English. In the tips and notes of this section in the web, there a some guidelines to understand this differentiation better. Both examples above can also be simple past.
I ate yesterday - (io) ho mangiato ieri.
I did not (or didn't) eat yesterday - (io) non ho mangiato ieri.
So, I had translated the sentence of the exercise as present perfect, it would be something like Let's say that we have not done it.
"we say that we have not made it" was what I wrote and it was marked as correct.