Thank you, t.winker. This lingot is from me, whatever the hell that is. For you especially, a star would be more appropriate. Just looked up the rule:
When a pronoun takes the place of the name of the indirect object, use the following pronouns:<pre>
me (me) te (you-familiar) le (him, her, you-formal) nos (us) os (you-all-familiar) les (them, you-all-formal)</pre>
Why would it be an indirect object? Another must know rule: Because entender is intransitive when the object is persons or animals.
A) entender, TRANSITIVE is to UNDERSTAND and has DIRECT OBJECT. The direct object form of the personal pronouns are like the indirect ones (see mick10...) except for III person
the direct object of él/ella/Usted is LO/LA/LE (also LE if a male in Spain) and of ellos, ellas, Ustedes it is LOS/LAS
B) entender INTRANSITIVE is KNOW = have a lot of knowledge or experience and has INDIRECT OBJECT
he knows a lot about wines, yes he knows them él entiende mucho de vinos, sí les entiende
AND DUO'S SENTENCE WITH THE DIRECT OBJECT LO IS OK
That would be leísmo, the verb is transitive in this context, if it weren't, it would need a preposition, either "de" or "en" to introduce the complement. If there were a direct object then the person would become the indirect object, for example: "No le entendí nada" (nada = DO / le = IO).
It will accept a wrong answer if (1) it is wrong by only one letter and (2) the mistake does not result in an actual word. It's just too hard for Duolinglo to correct multiple mistakes, or to guess what you intended when the result of the error is another valid word. People are better at this.