"They will stay at home."
Translation:Se quedarán en casa.
Hola amigo tipresn: Nothing. In fact, that is better than the Duo translation.
You can. Report as a correction. Duo seems ''married'' to the idea that you should always use personal pronouns although, generally, answers without them will be accepted as correct. In real life, personal prns are mostly used if clarification is needed or to emphasize yo, ella, ella, ellos, nosotras, etc.
"A casa" would imply movement. "Quiero ir a (mi) casa" = "I wanna go home".
Here they stay at home so it is "en casa"