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.

The difference between stai and sei in this instance, anyone?

buona sera is perfectly correct

Would it be buon sera in the subject and buona sera in the object?