Nothing wrong with saying ‘We are in January’ where I come from. Of course you can say it as ‘It’s January’ too but ‘We are in January’ implies something more like ‘It’s January right now’.
So, unless there’s some reason why the French sentence doesn’t mean the same thing, it’s a duolingo oversight.
“It is in January” in English doesn’t mean that we are currently in January today, right now. It means that something we are talking about is happening in January.
« Nous sommes en janvier » means that, today, now, we’re in the month of January.
So you can translate « nous sommes en janvier » as “It is January (now)” but not “it (the event we’re talking about) is in January”.
NB Where I come from, we say “We’re in January” in English all the time so it’s easy to understand the French. I didn’t notice until this course that we don’t also say “We’re in Thursday” but only say “It’s Thursday (today)”. But it appears that French people use the phrase for both days and months.
I find it easy to think of being in a month but not in a day but if you don’t then I guess you just have to learn the phrase « nous sommes jeudi » without the en. Prepositions are just one of those things that don’t always match up (English to French).