You have to look at the ending of a word;
Ending -consonant: masculine Ending -a: feminine Ending -e, -o, -um: neuter
Though Duo accepted "The food's good", it said I'd "missed a space".
That's what I thought it should be but I could swear she was pronouncing an 'a' at the end of the first and last words.
Again, version "A food is good" should be accepted here, as from the context we do not know if we are talking about this particular food or not. If we talk about food in general, then my version should be accepted :)
Food is uncountable. There is no such thing as 'a food', just like there is no 'foods' form.