If you are trying to create an invoice that referrences a Purchase order, and get the following error messages, please follow these steps to resolve the issue.
To resolve this issue, please consult the Purchase Order you are trying to invoice against.
When invoicing against the below Purchase Order, each line item in the invoice must correspond to a line item existing in the Purchase Order. As such, an invoice against the below Purchase Order must have Purchase Order Line item IDs "1" or "2". 
The below invoice will be rejected, as it has a "3" in the Purchase Order Line Number field: