Good question. 'Buona sera' is by far the most commonly used expression in Italy for 'good afternoon.' Of course, 'buon pomeriggio' translates literally as 'good afternoon', but it is never, or almost never, used by Italians.
I am confused. The correct solution on my screen showed up Buon giornata (something I'd never heard before). I used Buona sera as that is what I used when I traveled in Italy and everyone seemed to use. But then on this screen (where I'm entering the comment) it gives the correct translation as Buon pomeriggio. And there's this big red box that warns me to stop the clutter which I find a bit offensive.
I typed "bouna sera" as well, but I guess it translates litterally to "good evening".
I understand that 'buon pomeriggio' is not an everyday expression for Italians as it is England, Italians preferring 'buon giorno' or 'buona sera' as the time of day suggests. However I also understand that 'buon pomeriggio' is used on afternoon television.; perhaps native Italian speakers could comment.
As I understand it , 'buon' is generally used when it precedes the masculine noun, and 'buono' when it comes afterwords.
I use "buon medo giorno". Good half a day. It was marked wrong. Yet i have heard of it in RAI Italia. ???
What you probably heard was "buon mezzogiorno" which literally means "happy mid-day", or "happy noon." I have not heard this, but seems like it would be good Italian, even though it is not a direct replacement for buona sera.
My impression is that they are using "buona sera" all over the place, after 2 pm or 3 pm, depending on where you are in Italy.
In 10 years of living in Italy I have NEVER heard buon pomeriggio, After 12 noon (in my part of Italy) it has always been buona sera. To me, this is a serious error and should be changed. I can see that many have the same comment, and cannot understand why such a serious issue has not been addressed. Here, if you said buon pomeriggio you would stick out as a foreigner (or at least someone from some unknown part of Italy)