"It is true."
Here's a general observation about why Duolingo may reject a translation that seems obviously OK to the learner: Probably because no one says it that way, and if Duolingo told you your answer was correct, they would be encouraging you to speak Spanish like a foreigner.
Another possibility is that your answer is fine, but it didn't occur to Duolingo that someone would add an unnecessary word. However, after a question has been on Duolingo for a few years, Duo has probably added all the reasonable translations to their list of correct answers. "It is true" has been here for 5 years. Duo accepts "Es verdad" and "Es cierto." So apparently those are better translations, corresponding to what Spanish-speakers actually say.
Btw, the correct use of "lo" can be tricky. Example: I sleep enough. = Duermo bastante. = Duermo lo suficiente. "Bastante" can be either an adjective or an adverb. Here it's an adverb. "Suficiente" can only be an adjective, but "lo suficiente" together act as an adverb. :-)