Hello Jasim,
As far as i know the error is valid.
My understanding is when ever we set the cardinality 0..n or 1..n , we will have to specify the return entity set and this is needed when wet set the HTTP method to GET.
But with methods POST/PUT/DELETE, we would act upon on an entity and return type is expected is also an entity.In those cases we set cardinality 0..1 or 1 and we specify return entity only.
Regards,
Ashwin