Different purchase order supplier &1 planned for &4 &2

What causes this issue?

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

System Response

You entered an invoice with reference to &V4&.
In <ZH><ZK>&V4&</></><ZH><ZK> </></>&V2&, invoicing party &V1& is
entered. You are now posting the invoice to the invoicing party &V3&.
This may be for one of two reasons:
You entered <ZH><ZK>&V2& </></>in the field &V4&, and &V3& in the field
<ZH><ZK>Invoicing party</></>.
You entered a multiple assignment. The first item refers to &V4& with
invoicing party &V3&. Another item refers to <ZH><ZK>&V4&</></><ZH><ZK>
</></>&V2&. The system copies the invoicing party from the first item to
all items, in this case &V3&.
INCLUDE MM_IV_MELDUNGEN OBJECT DOKU ID TX

How to fix this error?

Check your entries and make any necessary corrections.
If you want to prevent items with different invoicing parties from being
selected, change the system message to <ZH><ZK>E</></> in Customizing.
To do this, you work through the step <ZH><ZK>Define Attributes of
System Messages</></> in the Implementation Guide for Invoice
Verification.
By default, this message is set to warning. In this case, all items are
selected.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.