Translation:There is no simple solution for that problem.
This is also correct in English:
There isn't a simple solution for this problem.
In general, can there be a way to submit potential alternative solutions where members can vote on their correctness (or downvote for their incorrectness).
I think that Duolingo cannot record all possible contractions, this is why the software has refused your translation. In the future, you may avoid them to secure your hearts.
Negative expressions drop the article after preposition "de": ne... pas de, ne... plus de, ne... jamais de...
the positive would be "il y a une solution" (there is a/one solution); plural: il y a des solutions (there are solutions).
But wouldn't "il n'y a pas une solution simple" mean "il n'y a pas une solution mais deux"?
we would rather say: il n'y a pas qu'une solution simple (implied: maybe several)
Rule: the usually required article is dropped in expressions of quantity: peu de, autant de, beaucoup de, un grand nombre de, plus de, moins de...
So I transkated "de" as "some", but DUO refused the answer, as in" There isn't some simple solution for that problem",Using, as an answer, an "a" instead...