Duolingo translation says: there is never enough milk, I wonder if the meaning of this sentence could also be: milk is never enough? - saying that you need always something else with the milk
I think you couldn't use the Czech genitive (mléka) for that. It literally says "of milk there is never enough." For your translation mleko would have to be in the nominative.
This is a wild guess, but maybe "Milk is never enough" would be something like "Mléko nikdy nestačí."
...but wouldn't you need to add something like "only, just, alone" to such sentence in English to get that meaning?
At least in the US, "Milk is never enough" would be understood as aososam describes. And it wouldn't surprise me at all if it were to suddenly turn into an advertising slogan for some brand of cookies!