Translation:There is a wide street under our windows.
I agree ! So frustrating to get a wrong answer when it is completely identical"
The wording you used is understandable, but it's not the natural way it would be said in English. What Duolingo offers is the natural way of saying that in English.
"A wide street is under our windows" was not accepted, and it suggests a solution of "A wide street runs under our windows."
I don't see anything explicit regarding the word "runs", would it be implicit here?
FWIW, Google translates both "wide street" and "broad street" as "široká ulice." I think in the US most people would probably use "wide," but there's nothing wrong with "broad." You might want to Report this if you get it again, since "broad" MAY just have been overlooked as a valid translation.