cannot/could not

I think both are correct. But I think "cannot" is a better answer.

Language is much more complicated that the simple rules, Tanguatlay. Why? Because life is complicated and there are many possible emotive responses that different speakers might/may want to express.

The fact that remains can/could be just as easily expressed by using either could not or cannot. Why? Because both can and could can/could be used to discuss future, present and past situations.

In this case, using could not makes the response seem, 1) less forward/more deferential; 2) a simple backshift that illustrates reported speech.

We can really tell whether the use of could not is 1) or 2) though we probably could if we were in the actual situation.