At this moment, child lists always require a parent list. Once a List attribute has been defined as a Hierarchical Attribute List (parent list has been added), it can’t be changed back to a regular list attribute. You need to delete the attribute and recreate it without selecting a Parent List.
This feature has been requested by various users. There is currently an idea in our Ideas Portal which proposes to add the possibility to remove the hierarchical association of attribute lists and it is currently classified as a future consideration. So, if you want to know about future updates, we invite you to vote for this Idea and follow it, you can access it by clicking on the following link Ability to deselect parent list in hierarchical attributes| Revvity Signals Software Ideas.