I formed Arnor right now, but I can't recruit the Arnor units. Do I have to destroy my barracks and build them again?
Just wait a few turns. They have a different culture, I think, and you will need to wait for conversion.
Charger is right Temudschin. In general the moderators encourage you to ask that kind of question in the "General Questions"- thread in order to keep the Forum uncluttered.
So, if you want to not annoy them and thus not get annoyed by them; you should always check the FAG, General Questions, and lastly do a search to see if the question is answered elsewhere. Then consider where it belongs, before posting.
It is very human to just post and I know I sound like a schoolmaster, but that way is easiest for all and avoids clutter.
Thanks for considering it.