I have taken ownership of some hosted data that has a relationship class to a table.
I assume this is because of the Relationship? I go to another Hosted Feature Layer that does not have a relationship and I can see the Add Fields button.
If that is the case how to I add a field to a Hosted Feature Layer that has a relationship?
I just tried this and the Add Fields button is visible for me for a hosted feature service with a relationship class. By any chance is your hosted feature service restricted from being able to add fields?