"She likes to run."
Translation:A ella le gusta correr.
Well, maybe it's a Duolingo mistake but only "le" is really required.
Unlike the verb to like in English which takes the person as subject and the liked object as DO, the Spanish verb "gustar" takes the object as the subject of the sentence and the person who likes the object is the IO.
But as long as you put the "le" you don't need to add "a ella" although it is common to use both. If Duolingo doesn't allow to drop off the "a ella" maybe it's because it doesn't have it as correct solution. It is simply a mistake.