ни- или не- in negative pronouns

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

The last one is wrong. You cannot use "не с кем" in such sentences, only "ни с кем" ("Ни с кем нельзя гулять" is OK).

"Мне (тебе, ...) не с кем" means something like "I/you/... don't/doesn't have anybody to ... with."
E.g.
Ему не с кем играть. - He doesn't have anybody to play with.
Ей не с кем поговорить. - She has nobody to talk with.

And "Мне (тебе, ...) нечего" means something like "I/you/... have/has nothing to ... ."
E.g.
Мне нечего сказать. - I have nothing to say.
Мне нечего принести. - I have nothing to bring (with?).

Hope it helps.

Thank you for your help.