"Your lemon cream is sour; it's delicious."
Translation:Ta crème au citron est acide, c'est délicieux.
I think Duo is wrong and it should be elle est délicieuse since it's just an adjective describing a thing.
It seems « aigre » has more of a negative connotation; see the discussion here:
The word "sour" in English also traditionally has a negative connotation, and would often be softened with another adjective to express otherwise for example "pleasantly sour". Whereas (at least in America) the English word "tart" would be used to express that something is sour but tasty without a necessarily negative connotation.
So, when talking about food, maybe "tart" is closer in meaning to "acide" and "sour" is closer to "aigre".