Translation:This tea is spicy, isn't it?
The ね is not exactly equal to "isn't it?" Instead, it's a sound used when the speaker thinks the listener will agree with the sentence. So it's like the italian "no?" at the end of an Italian sentence, or a USian saying "right?". The Japanese course creators have decided to translate ね as "isn't it?", so even though your answer is technically the same, it's not accepted as correct.
And as a USian, if I heard someone saying "is it not?" I would be certain that they were not a native English speaker.
I'm not sure I understand why "This tea is salty isn't it?" would be the answer. からい is supposed to mean spicy isn't it? So why would it translate as it being salty?
It seems the word からい originally meant "salty" and later developed the sense "spicy". (Or, perhaps, it meant "spicy" in a sense which encompassed "salty", with saltiness being the most commonly salient kind of spiciness? But I'm speculating here.) New words came into use with the specific meaning of "salty" (しおからい (literally "salt-spicy") & しょっぱい) and these days からい has the basic meaning "spicy". However it is still sometimes used to mean "salty" (perhaps more commonly in some regional dialects than in standard Japanese).
On Duo, "Spicy" (辛い) is sometimes pronounced "Karai" and sometimes "Tsurai" (which I think means "Difficult"). Is this deliberate, to stress that something it's so spicy that it's difficult to eat, or is it just that the system picks up a random audio sample that matches that Kanji?