If it's the same as in Swedish (which I'm assuming since the languages are very related with few exceptions compared to e.g. English and Swedish or French/Spanish/Italian and Swedish), then genug = is there a sufficient amount of [something]; does the quantity reach the needed amount/sufficiently cover our needs [for something].
You can use either or most of the time depending on the context, but they change the structure of the sentence.
The Swedish word "räcker" (GER: "reicht") is a VERB (ENG: "to be enough for/reach [something]"; "to suffice"), while the Swedish words "tillräckligt"; "nog" (GER: "genug") are ADJECTIVES or ADVERBS (ENG: "sufficient", "sufficiently").
SWE: "räcker"/GER: "reicht" is often something that IS; a state of BEING.
— "That IS enough [of something]".
SWE: "tillräckligt"/GER: "genug" is often something you HAVE.
— "I HAVE enough [of something]".
The sentences "Det räcker!"; "Det ska räcka till alla" are best translated as: "That's enough!" (can be used to chastise someone, i.e. "Quit it! Stop!"); "It needs to/is supposed to be [enough/sufficient/the required amount to adequately reach a set extent] for everyone".
Compare to the sentences "Det är tillräckligt!"; "Jag tror vi har tillräckligt nu", which are best translated as: "That will suffice [be enough of something]" (can be used as "Don't add more!"); "I think we have enough [of something] now".
Okay, usually when Duolingo does the whole "Pick the word that fits in the blank" thing... there is only one option that fits and is grammatically correct. And while the option "gerne" produces a rather odd sentence: "Do we like having milk?" ...it's still correct... isn't it? ...It's something one could ask. Despite there not being... many situations where one WOULD. I mean sure it's obvious now that genug was correct... but come on... bearing in mind that it had been doing nothing but "gerne" questions before... how is that fair? >.<;