The correct code to enter is: give_trait "Caivsavrelivs Cotta" "Popularis" 1

See, you've got the name thing that's difficult but also for the Romani when you get "Africanus", for example, the database still registers the character under the original name. Here, I assume you're talking about Caivs Avrelivs Cotta, the guy you get in Arpi at the start.

This brings me to another point. Did you change the date requirement for the marian reforms? Normally, you need to have gone past 175 BC (IIRC), so if you've got the original Cotta, chances are, you're not gonna get the reforms.

Hope it helps.