It's a bug. I suppose there has been a "flash flood" once in this province. This makes it impossible to construct anything in the field as well as effectively destroys any roads/paved roads/highways. You can still see them on the map but they have no effect anymore. Does you General have a limited movement range in that province? If not, then it might be that there is a strategic unit (spy/diplo/assassin) in an adjacent tile to your generals unit. They prevent field construction also which I consider to be a bug as well.