Jace, Telepath Unbound and Casting a Spell from the Graveyard
I don't know how MTG Duels exactly works but my guess is this, the card says


  You may cast target instant or sorcery card from your graveyard this turn


Which means that you are able to target a card in your graveyard but decide to not cast it. After this ability resolves you still have to manually cast it from your graveyard. You have the entire turn for that. It doesn't automatically get cast when the ability resolves.