Цитрина means lemon in a certain dialect (Галицький діалект). But we don't accept dialectal words here, otherwise we'd have to add ten words for "lemon", and probably for each word :) And because it's more useful to teach the standard most mainstream dialect. But, it's fun to learn dialectal words in the comments! ^^
лимона is the correct form here indeed.
And you are right, one could probably say that countable nouns usually take -а/-я, while most uncountable nouns take -у/-ю. This definition would be helpful but it would also have a number of exceptions (like names of cities which are uncountable but take -а/-я). I think I'm going to make a detailed post about that :)
Exercises in this lesson keep stating хліба, and Vinnfred's comment above states that the proper ending а/я or у/ю has nothing to do with countable or uncountable nouns. My understanding to date is that a lot of Ukrainian grammar revolves around what sounds good/better. Maybe that's the case here; хліба sounds better to me than хлібу.
This confusion is for masculine nouns with no vowel at the end in Genitive case.
The most general rule is, if it's an object, if you can touch it, see it, it's а. If it's abstract, or uncountable, or a mass noun, it's у.
But, there are exceptions. E.g. math terms all have а (косинуса, синуса) even though they're abstract.