"اَلْسَّلامُ عَلَيْكُم! وَعَلَيْكُمُ ٱلْسَّلام!"
Translation:Peace be upon you! And upon you be peace!
You dont have to translate this literally its just like saying hello in a nice way nobody really thinks about the meaning
Duolingo is too picky about the translation for this. For example, "and upon you be peace" sounds less natural than "and upon you peace". But even beyond that the way to express this in English is "peace be with you" full stop.