When deleting a feature from an origin feature class in a simple relationship, the following error may occur:
"The features could not be deleted. Failed to delete relationships for object. [GIS.Address] The field is not nullable. [STREET_NAME_ID]"
This error occurs if the foreign key of the relationship has a Not Null constraint. When an origin feature is deleted, the relationship it had with the destination feature must be broken. This is accomplished automatically by setting the value for the foreign key to Null.
Remove the Not Null constraint from the relationship's foreign key. See: Modify field properties.