"There is not too much milk."
Note up front: it's 'mattaku' (not kitaku), and 'ookunai' is a single word.
As for why they're both here: the former qualifies the latter. まったく means "entirely/thoroughly" or -when used with a negation- "not at all". おおくない is the negation of おおい, meaning "a lot/many/plenty". So together it's like saying "there is definitely not much" (I'm personally not fond of Duo's choice of translation; "not too much")
全く/まったく/mattaku is an intensifier or qualifier. It's fairly strong. quite/absolutely/entirely/completely/etc. I see it written in both kanji or hiragana. It's hard to given one comparative English word that would apply in all uses.
Leaving it out makes the statement a less forceful assertion.
まったく違うよー 。まったくちがうよー。mattaku chigau yo-
That's totally wrong! (You're way off base!) It's completely different!
違うよー 。ちがうよー。chigau yo-
No, that's wrong! That's not right!
全く期待はずれ。 まったく きたい はずれ。mattaku kitai hazure
It's completely disappointing. It totally didn't meet expectations.
期待はずれ。きたい はずれ。kitai hazure
It's disappointing. It doesn't meet expectations.
It can be used with positive or negative phrases.
It can also be used an exclamation (though it is not used in this fashion in this course). It's along the lines of expressing distaste, exasperation, surprise, disgust. Sort of like the English exclamations "Seriously?!" "Honestly?!" "Really?!" "Good grief!"
See Alcedo-Atthis' explanation further up in the discussion. It discusses some of this, too.
Alcedo-Atthis commented that the translation Duolingo offers is kind of odd; I agree that it makes it harder to understand why you'd want to say this, given the suggested translation.
If you think of 'mattaku' as an intensifier, and 'ookunai' as not much, you can get the slightly more useful translation as 'definitely not much'.
Imagine the following scenario -
A: I wonder if there is enough food for the party... B: There is definitely not much food here! (You probably want more.)
A: パーティは食べ物が充分あるかなぁ？ B: まったく多くないよー
A: パーティは たべもの が じゅうぶん ある かなぁ？ B: まったく おおくない よー
The intensity of the added まったく makes it more imperative you'd want more food than what's present.
As IsolaCiao mentions, the translation offered by the Duolingo Japanese staff does make sense from a native Japanese speaker's point of view. But the English is a bit odd.
I think this is a great example of why thinking about Japanese in terms of English is ultimately counter-productive in the long term.
If you can get into thinking about Japanese in terms of Japanese, you'll be better off.
But it's totally natural to try and speed up your processing of a new language in terms of the language you understand the best; you've got to start somewhere!
I know it's not the dictionary definition, but I agree with duolingo, in Japanese daily life "ooi" means "too much", so "ookunai" would mean "not too much".
I think jessfitzgerald gave a great explanation of the nuance of using "mattaku" or not using it. Clearly the English translation here doesn't do anything to signal whether we should use "mattaku" or not.
多くないです(ooku nai desu) and 多くない (ookunai) mean the same thing. Use of です 'softens' the phrase, and makes it sound more polite. ない is a 'rougher', less formal word when used without です.
Textbooks and teaching often default to using polite conjugations.
This duo course does use both, which is good - it's helpful to be exposed to various levels of politeness and conjugation. It is not always consistent about accepting both, which is not so good. It also doesn't explain why and when you'd want to use one versus the other, which also isn't very good.