There are still payers defined at item level

What causes this issue?

<GL:payer>payers</> still exist at item level which differ from the
payer entered at header level.

System Response

INCLUDE 'NA_NO_PROCESSING_2' OBJECT DOKU ID TX LANGUAGE D

How to fix this error?

First change the payer at the corresponding item level, before changing
the payer at header level.

Error message extract from SAP system. Copyright SAP SE.