Translation:This tea is sweet.
I get what you mean (where the emphasis is on the adjective, like "this is a good song") but that's sort of a colloquial thing.
Duo's being strict about sentence structure to make sure you understand the difference, so it's safer to reject the 'incorrect' version (even if people do use it in place of the 'correct' one, with the exact same meaning)
I'd be curious if any native Japanese could give me some perspective here...if someone said a sentence like this, would it be more likely to mean that the tea had been sweetened, such as with sugar or some other sweetener? Or would it be more likely to mean that it was just plain tea that naturally tasted sweet?
Or could this sentence be used in both contexts?
I had sweet milk tea. Tasted like milk in bowl after you eat the cereal.