Supplier & for PO & replaced by &

What causes this issue?

This message is issued in three cases:
On the initial screen under "Selection" you entered a supplier as well
as a purchase order, a delivery note or a GR document, and this supplier
is not the same as the one from the purchase order;
On the multiple selection screen, if you have entered several purchase
orders, delivery notes or GR documents that do not all belong to the
same supplier;
During processing, if you reference a purchase order that does not
belong to the supplier of your current processing (via the menu <LS>Edit
</> --> <LS>New item</> --> ...".

System Response

The invoice will be assigned to
the supplier you entered on the initial screen, or
the supplier in the first purchase order in the multiple selection.

How to fix this error?

You cannot change the supplier. If you do not want to assign the invoice
to this supplier, you must first acknowledge the message and then
reenter a document.

Error message extract from SAP system. Copyright SAP SE.