If you are unable to add a successor guardian, it could be the following reasons:
You have already selected the individual as the initial guardian, therefore that person cannot be named as a successor.
You have not yet added the individual you want to select in your "Family and Contacts" section.
The person you are trying to select is not 18 years or older.
If any of these conditions are met, you will not see the name of the person you are wanting to select from the drop-down menu.