3171900 - Unable to change some fields on a change Purchase Order or Requisition

Why am I receiving the below error on changing some fields on a change Purchase Order (PO) or Requisition:

Cannot submit, save, or approve this requisition because of one or all of the following reasons:
1) Change in the requisition may result in the deletion of line item [x] from order [y], and subsequently lead to the creation of a new order. This is not allowed.
2) Change in the requisition may result in the cancellation of order [y]. This is not allowed.

accounting value accounting data accounting field , KBA , migration source: knowledge@ariba , migrated on: 2022-04-02 14:02:06 , error messages , change requisition , requisition accounting , BNS-ARI-PUR-REQ-IEX , Requisition Import/Export , Problem