"You send the songs and the letters to Marcus."
Translation:Carmina et epistulas ad Marcum mittitis.
Perhaps Duo does not accept the dative version because Duo feels it hasn't taught us that, yet.
But I do find it more intuitive. Actually, for a moment I thought that "Marcum" might be dative and "ad" might be unnecessary or clunky, so I (mistakenly) omitted "ad", like saying in English "You send Marcus the songs and letters".
Well, my guess is that ad is more specific as into or to whereas the dative could mean other things, but you have a point since the dative is an indirect object, not the direct object whereby it must take the accusative. In addition, mittis seems acceptable to me if "Tu" or "Vos" is not included.
As ClausFenge said, the pronoun would not normally be used here. But Duo does normally accept answers with pronoun subjects.
What was your entire answer? If you used "tu" but also the "mittitis" verb form, that's ungrammatical because "mittitis" is the "vos" form and not the "tu" form. Correct would be "Tu carmina et epistulas ad Marcum mittis"; if that's what you wrote, it should have been accepted.