"They have to pay for the fruits."
Translation:Ellas tienen que pagar las frutas.
I don’t think it’s wrong, but if they had accepted it, you may not had noticed that you also could have omitted it. pagar can mean pay or pay for. you use por, when paying for someone else, for something you did (like with a punishment) or for something you keep (like a house). I think fruits are borderline. Also notice, even in English we don’t always use “for” e.g. We pay the bill.
Why not Tienen que pagar para las frutas? I was marked wrong for this answer.
Tried it without "ellos" or "ellas" but that it not accepted either. Duolingo is ONLY accepting the answer with "ellas."
Google translate confirms that I am correct using Ellos tienen que pagar por las frutas
Pagar means 'to pay for'. so the 'por' is not used. Don't take Google Translate too literally.
This lesson contains some form of "tener que pagar" without "por" numerous times so I assume "tener que pagar" = "to have to pay for". Adding "por" would be incorrect and redundant.