I think you should add public each property.

example)

[Serializable] public class Mission { public Mission() { Parent = -1; }

[SerializeField] public int Parent;
[SerializeField] public int Id;
[SerializeField] public string Name;
[SerializeField] public string Description;
[SerializeField] public MissionState State;
[SerializeField] public MissionType Type;

}

I think you should add public to each property.

example:

[Serializable]
public class Mission
{
    public Mission()
    {
        Parent = -1;
    }

    [SerializeField] public int Parent;
    [SerializeField] public int Id;
    [SerializeField] public string Name;
    [SerializeField] public string Description;
    [SerializeField] public MissionState State;
    [SerializeField] public MissionType Type;
}