Because "Aucun" requires a noun like "un"
"Un chat", "Aucun chat" (a cat, no cat)
If you say "Je n'ai aucun", a French native will ask you "Aucun quoi ?"
So you should say "Je n'ai aucun chat" (I have no cat)
In the sentence, the thing you are talking about is implied because it was previously mentioned: (Do you have a cat ? I have none.)
Since the noun is implied, you have to use a place holder which is 'en'.
"As-tu un chat ? Je n'en ai aucun."
Oh yeah! En is used in a variety ways for that purpose:
In English, when you say "I don't have any", it is usually understood what you are talking about as you are probably answering a question like "May I borrow __?" or "Do you have _?" So everyone understands that you are saying you don't have any (of it/them), whatever _ is.
The French negation is built with exactly two things: "ne ... kind of negation". The ... is where you put the verbs or verbal form (verb + adverb)
The kind of negation depends on the nuance you want to add:
- ne ... pas (plain negation)
- ne ... aucun (specify absence of countable thing)
- ne ... plus (timed negation, "use to have but no longer")
- ne ... jamais (timed negation/rule, "do not have and never had"/"Not allowed")
- ne ... rien (specify absence of uncountable thing)
- ne ... personne (specify absence of a human being)
- You cannot omit the 'ne' (although a lot of native speakers do...)
- You cannot have more than one kind of negation (child-speak/incorrect)
- You can however "stack" some kind of negation to stress things (but they must stay together)
- "plus jamais"/"jamais plus" (never ever again)
- "plus aucun" (nothing more, countable)
- "plus rien" (nothing more, uncountable)
- "plus personne" (all are gone, were there but no longer, humans)
- "jamais rien" (always absent, uncountable)
- "jamais aucun" (always absent, countable)
In your case, "pas" and "aucun" cannot be stacked because "aucun" is more precise than "pas" so it is pointless to stack them: just use the stronger "aucun".
Well, there are many ways to get to Rome. The answer given may not be an exact word-for-word copy of what you see as the solution but I bet my bottom dollar, one of the choices echoes the sentiment of the given answer.
Look at these phrases:
I don't know;
I have no clue;
I haven't got the foggiest idea;
Color me clueless...
Are all different ways of saying the same thing. Language is colorful like that.