"Good afternoon, how are you?"
Translation:Buon pomeriggio, come stai?
Duolingo is not teaching dialect, or local custom. Buon pomeriggio is the literal translation. Whether or not it is commonly used in a certain region or province is of no signifigance in this type of setting.
Also "buona sera" or "buonasera" should be quoted among the correct answers; in the (late) afternoon it's correct to use this expression.