How Does the 'Approve and Forward' & 'Forward' function works in Purchase Order Approval?

We are using 'Position Hierachy' method for Purchase Order approval process. During approval process we want to forward the Purchase Order to the requestor (Purchase Requisition`s requestor) as well to get approval. We cannot add the requestor in the approval hierarchy since there are 'N' number of requestors in the organization. So during approval process upon sending to the 1st approver, the 1st approver selected the relevant employee name (requestor) and clicked on 'Approve and Forward'. But its stoped with the newly added persons and displayed 'No Approver Found' and some time its end with some error.

Could you please tell us How Does the 'Approve and Forward' & 'Forward' function works in Purchase Order Approval?