Cannot Add Delivery from Sales Order

Hi,

I am trying to add a delivery document by using the 'copy to' option within the sales order document but get the following error message: (1) Unit price for Sample items should be zero. Kindly Update!

This error message comes up when I press the 'Add' button on the Delivery document. I am using SAP Business One Version 8.81 PL4

Any ideas what should i be checking?

Thanks.

albert