Translation:They are not interested in playing tennis.
Agree. Report it. The infinitive (here it's 'to play') is what's being conjugated in almost every verb. It is the basic and correct verb form and should always be accepted by Duolingo for that reason. The infinitive phrase here ('to play tennis') is also the direct object in English (cf. 'to take a walk', 'to take swimming lessons'). Personally, I don't think there's anything unnatural in the phrasing, nor any obvious loss of meaning, so again it should be accepted by Duolingo.
You make an interesting point, but while Laura's sentence would be understood, it's a construction seldom, if ever, heard coming from a comfortable English (US) speaker.
We will always have some tension here as to how far Duo should go in accepting answers that may be technically correct but are, as nEjh0qr4 says, unnatural English. This brings us to the never ending question as to which is more important being, marked "correct" or learning the language(s).
Not in this case. Encantar and gustar and odiar all take this form and you would use les (or nos for us or te for you etc) and never las.
However if you have a sentence like: These letters, I am going to read them.
It translates as: Estas cartas, las voy a leer.
I think so, at least. And now I can't figure out what the difference between the two cases is.
Randall, you've probably learned the answer to this in the 5 months since your question, but in case not . . . . A ellas means "to them." To say "to the girls" you would need a word that means "girls," for example a las chicas . . . . You could say that, and the phrase would be even more clarifying than a ellas. But, that's not Duo's prompt, here.
Thanks for your perspective. I thought that "a ellos" (like los padres, los abuelos, los ninos) was used for the plural (as well as for "the men"). Fully agree, that using "chicas" is best for clarifying. This is 1 aspect that irritates me about Duolingo; sometimes it's very loose in its translations & other times very strict. But it's a lot better than most for the cost!!
Pablo, I wish you had retyped the sentence you began "a nosotros". We can't compare two things when we can only see one of them.
But, whether a ellas is appropriate to begin the sentence depends upon the verb. For example, here interesar is being used like "a verb like gustar".
For "verbs like gustar," see:
http://www.spanish411.net/Spanish-Using-Gustar.asp https://studyspanish.com/grammar/lessons/gustar (this one has a list of such verbs)
Because "the girls" is "las niñas", or "las chicas" (or whatever other synonyms). "Ellas" is just "they".
Yes, that means you lose the gender specificity when you translate to English, so I can understand the desire to specify "the girls". But losing that gender info is a normal part of translation, just because of the way English and Spanish work differently, and is generally okay. Like just translating "jefa" to "boss" instead of "female boss"; or translating "her car" to "su carro" (losing gender information in the opposite direction).