I see in the Grammar that in negative pronouns these prefixes are used:
ни- in 1) personal phrases and 2) to confirm a denial.
Я никого не вижу
не- in 1) impersonal phrases and 2) to express a negative.
Ему нечего читать.
The exercise deals with giving a negative answer to the questions.
С кем можно гулять?
Ни с кем нельзя гулять (and I confirm the denial existing in нельзя) or
Не с кем нельзя гулять (It is a impersonal phrase). I don't see nowhere an example with this "impasse". What do you think of it? Thanks