This error is typically caused by an issue with a purchase order linked to a client within the invoice period. The known causes are:
A purchase order where the start time and end time are different but the unit duration is set to 0.
A purchase order where the start and end date are the same.
A purchase order with no cost or charge per unit set.
Check the purchase orders linked to the clients within your invoice period. If you need assistance, raise a new case online and reference the title of this article.