Check the relationship and custody information recorded in the relationships tab and ensure that it is accurate.
The error sometimes may occur not because the last name and first names fields are blank, but because no one was given “custody” for the student(s). In other words, if the student does not have a parent (mother or father) designated in the relationship field it is ok. However, if that is the case, then at least one person (relative, guardian, etc.) must have the “custody” field designated with a “YES”. A “YES” under the custody field is not a “legal” designation according to RenWeb; it is just another filter.