The most common reason why you cannot assign a safe area to a user is that they do not have a GPS device attached to them. In this case, there is no button to add safe areas as these would not work. If the button to add safety areas is on the user but the safety area you want to link is not in the list, it is because that particular safety area is not supported by the device the user is using. More information about this can be found here under Help & Support.