Thanks for attaching a screenshot. It really helps to illustrate the question you have.
What you are doing when you type the name in this box is creating a connection between a person's site member profile and their entry in the family tree.
I looked at your family trees and I wasn't able to find your cousin's name in any of the trees on your family site. In order to show the relationship of your cousin's site member profile to the family tree that person must exist in one of the trees on your family site.
Try adding your cousin to the tree first. Then you'll have no problem.