Adding a notification recipient in the flow builder

Hi! I am trying to create a custom notification flow but am getting errors when populating the recipient ID input value. 

When trying to add new fields, it takes me into a drop-down loop and this error message refrains me from saving flow:

"The 'x' field doesn't exist on the 'Case' object, or you don't have access to the field."

Has anyone else faced this issue or knows the solution to adding a recipient to a custom notification for Owner --> [EmailMessage].Parent.OwnerId ?

Thank you!