Exception While associating a record of setting up relationships (N:N) between child and parent records same entity

Code Fix :

associateRequest.Relationship.PrimaryEntityRole = EntityRole.Referenced;

AssociateRequest associateRequest = new AssociateRequest();

            associateRequest.Target = new EntityReference("account", childAccount.AccountId.Value);

            associateRequest.RelatedEntities = new EntityReferenceCollection();

            associateRequest.RelatedEntities.Add(new EntityReference("account", parentAccount.AccountId.Value));

            associateRequest.Relationship = new Relationship("account_parent_account");

associateRequest.Relationship.PrimaryEntityRole = EntityRole.Referenced;

            var associateResponse = xrm.Execute(associateRequest);