No se puede devolver un RelationshipManager para este objeto. Solo se puede devolver un RelationshipManager para los objetos de los que ObjectStateManager realiza un seguimiento o que implementan IEntityWithRelationships.
Click to search this error in Google
English version
A RelationshipManager cannot be returned for this object. A RelationshipManager can only be returned for objects that are either tracked by the ObjectStateManager or that implement IEntityWithRelationships.