Is it possible to cancel the purchase order line and then serialize the part and then re-enter the purchase order line?

not desirable, and we changed many other parts that were on a purchase order line to serialized without this error, it is only happening on two parts, odd.

@AnnieeMariee Even though there is no available quantities of this part in the inventory, you could get this error when the PO is in Received status.

The part exists on a Purchase Order Line

If the PO is still in the Released status and there is no other POs in other status, then I think this is not right. Please check.

Hope this helps !