I read and talk about international affairs.
It's common to say that a region, country, or even the world is at peace. (Unfortunately, it's also common to say that a place is at war.)
No-one would say this except in Duo's class.
Don't go down there that country is not at peace. Rejected. How so?
I had the same, and decided "down" was not wanted.
Whilst the phrase" at peace" is perfectly acceptable as indicated below ;the usual opposite is" at war "not " not at peace". That's why you get the double negative, the grammar is wrong
Don't go there, that country is not peaceful - rejected. One would not say "at peace" to refer to a country in English, "at peace" would refer to someone's state of mind (or if someone had died after painful illness).
I agree, this is a very unusual english sentence. It would be more usual for someone to describe a country as 'at war', but never 'at peace'.
in peace is accepted, is it better than at peace ?
Both are good. Maybe "at peace" is more specific to two parties getting along, and "in peace" is more a general state or condition.