"Thirty years is not much."
Translation:Trzydzieści lat to nie jest dużo.
"Trzydzieści lat to nie dużo" seems acceptable, added.
"Trzydzieści lat nie jest dużo" won't work. As for why... I'd say that it's because on the right side of "jest" (Y position) it is neither a noun phrase nor an adjective? Although it's also hard to imagine an adjective there... Anyway, "Trzydzieści lat nie jest długim okresem czasu" is maybe an a bit surprising sentence, but grammatically seems correct. But in the construction here you need this "to". Maybe someone can explain better "why".
What is sometimes frustrating with Duolingo is that you are required to translate something without any hint whatsoever on how to do that. In the case of this sentence, it's never explained that you should use either "to" nor "dużo", with the suggestion being "niedużo". So it's an incredibly confusing experience.
It's almost always because of the bug which makes multi-part hints always display on top, even if they do not apply to the given sentence. This may make the actual applicable hint not display, because not more than 3 hints are displayed.
I just created a hint "to nie jest dużo" on "is not much", so it should be on top soon.
"niedużo", however, is also correct. It's not a word we teach, but it itself means "not much". So you could go with "to nie jest dużo" or "to niedużo" here.