"They are not interested in playing tennis."
Translation:A ellas no les interesa jugar al tenis.
The answer shown in Duolingo with ellos replacing the ellas is indeed accepted as correct in Sep 2021 - Ive just this minute tried - so it must have been something else that was a mistake for you Carla.
Ive had moments when I thought I'd typed in the right answer but on closer scrutiny, Ive always found a mistake somewhere.
You likely were counted wrong on this question for leaving out the preposition 'a' before ellos, not leaving out the al after jugar. The 'a' is mandatory if you use ellos, but Duo has recently been making the al related to jugar optional since so many native Spanish speakers have been complaining that they don't actually use it.
But, to answer your question, jugar requires the 'a' when it is followed by a game, and sports require the definite article before their name in most cases. Here, that 'a' and 'el' combine to make 'al.'
There's a good response from lazarus1907 here:
Interesar is a "reverse verb" (my term) like gustar. Playing tennis does not interest them. Interesar conjugates for the playing not for them. That is also why you need "a" ellos, not just ellos. Other "reverse" verbs include aburrir, fascinar, bastar, importar, caer bien (mal), dar asco, molestar, disgustar, paracer, doler, picar, encantar, quedar, faltar, volver loco. There may be others. You just have to memorize them.
I have had real trouble completing all the levels in this lesson. I'm SURE Duo tells us one rule, then changes it! Eg. "A" should only be used when being specific about a person, like when you use their name. But now they've decided to mark it wrong for "they" too! I get sooooooo confused sometimes by them changing the grammar rules!
It's actually a lot more simple than it seems! If you are playing a sport, you always need to put an "a" and then the article in between the verb jugar and the name of the sport. So if we're talking about playing tennis, it would be:
jugar + a + el + tenis
Then, because "a" always combines with the masculine "el" to form "al," it's going to contract down to:
Jugar al tenis
"A" does not combine with the feminine "la," so if you run into a femine sport, it would be:
Jugar a la [sport]
The way it was explained to me many years ago... Ser is used in basic inherent cases, things that are not likely to change. Estar is used when it could change. The difference between, "They are not interested in playing tennis" and "They are not interested in playing tennis right now". In the first example, they are not interested and that is not likely to change.
Because they find something interesting and they aren't interesting. E.g. "Ellos les interesa el fútbol." They are interested in soccer.
But "Ellos interesa el fútbol." Is just wrong. "Ellos interesan." Would mean "They are interesting." "Ellos interesan el fútbol" would mean "They are interesting soccer."
So it is "(A) ellos les interesa el fútbol"