Yes, the correct form is лимона.
I guess not only do the creators of these exercises not know English well, they also don't speak Ukrainian very well xD And apart from that, they are overly confident about their knowledge because they somehow didn't doubt it, didn't have a second thought "Hmmmm.... Is it лимону? I should probably check". Because if they did, they would have seen the correct form on any website they would've clicked on...
The correct version is, of course, accepted :')
I, too, expected the answer to be лимона, and, in fact, that's what Google translate uses. Why does Duolingo give the answer as лимону? Does the rule say to use the genitive case with "без," and then it's a philosophical question if the masculine лимон is countable/concrete to take the -а ending versus material/substance/abstract to take the -у ending?
Some vocab for the tea lovers here!
Herbal tea -> Трав'яний чай
Mint tea -> М'ятний чай (or actually we just say м'ята, simply mint, we don't call it a tea)
Raspberry tea -> Малиновий чай
Green tea -> Зелений чай
White tea -> Білий чай
Black tea -> Чорний чай.
Sweet tea -> солодкий чай
Unsweetened tea -> чай без цукру
Strong tea -> міцний чай