Requisition &, item &, created from production order/network

The selected requisition item was created via a production order or
network. However, the already-generated purchase order item already
contains a requisition item created from a production order or network.
It is not possible to adopt more than one purchase requisition item of
this type in a PO item.

How to fix this error?

You can adopt this purchase requisition item in a new purchase order
item.

Error message extract from SAP system. Copyright SAP SE.