BLL Return String Or DTO

I'm confused how to return the result from the business layer. Sometimes I need to return the message if it does not pass the criteria. For example:

public SalesDTO GetSalesByPrescriptionNo(string prescriptionNo)
{
    int count = unitOfWork.SalesRepository.GetNumberOfPrescriptionUsed(prescriptionNo);
    if (count > 5)
        // I cannot return string/error information 
        // since the function is return SalesDTO type
        return "Cannot used anymore";

    var sales = unitOfWork.SalesRepository.GetSalesByPrescriptionNo(prescriptionNo);
    var salesDTO = Mapper.MapToDTO(sales);
    return salesDTO;
}

Based on the good OOP/OOD implementation, how should I handle the multiple result from the BLL?

Thanks in advanced.