I'm coming late to this party, but -- for what it's worth -- I don't think that "We do not have enough milk" is a good "main" translation for this exercise, unless it's the only thing that's really "right" from the Czech side.
As others have suggested, I would read "Máme málo mléka" as "We have LITTLE milk," and would expect to see "Nemáme dost mléka" for "We do not have ENOUGH milk."
The fact we have "little" milk doesn't necessarily mean that we don't have "enough." While it might not be much milk, it could be "enough" for the two cups of tea that we're making, for example.
Maybe one of the contributors/volunteers will explain...?
"a little" though is correct when referring to noncount nouns, such as money, food, time and many others. For example: I have a little time now to help you. (This is a positive statement, meaning I have some time.) I have little time now to help you. (This is a negative statement, meaning I don't have much time.) How would you say, "I have a little milk in the refrigerator," meaning some but not much? Thank you for your very quick and thoughtful comments.
'I have a little time to help you now' is a positive statement. It means that the help provider has time now and is willing to give up that time. 'I have little time to help you now' is more of a negative statement and suggests that the help giver does not have or is unwilling to give up what little time he may have in order to assist
If you read this page more carefully, you will see that ion is well aware of the distinction between "little" and "a little", and that both can be correctly used with uncountable nouns. See for example ion's first comment on this page (The second comment from the top).
As for the Czech, the meaning here is "he has little = not much milk".
Which is not what the Czech sentence means and why the official translation has "not enough".
But I do not want to re-ignite the unfortunate discussion in the reverse translation (https://forum.duolingo.com/comment/25031552) so I will not continue if any attempt to reignite it appears.
An English speaker can certainly say both "I have little milk" AND "I have a little milk; ion1122 has explained the difference between them above.
What's most important here, though, is that the meaning of the Czech sentence -- whether we understand why or not -- is closest to "We do not have enough milk," the translation now (7 Nov 2018) shown at the top of the page. We need to accept it.
Nemáme dost mléka. and Máme málo mléka. have slightly different meaning in Czech.
The first one says you do not have enough to cover the needs, while the latter says you have a low volume of milk (eg. in comparison to what you normally have, but it can still be enough for your needs).
Yes, I understand that the two sentences mean the same in Czech - but in the English version "too little" = "not enough", it's not an intensifying "too" in this case in this case - it's saying there isn't enough milk to spare. So it seems to me that "We have too little milk" is as good a translation of the Czech as "We do not have enough milk" and in fact adds fewer "extra" English words than the latter - and keeps the sense of malo.