Partner function & is not defined at item level

What causes this issue?

Header partners cannot be entered at item level.
INCLUDE 'NA_NO_PROCESSING_1' OBJECT DOKU ID TX LANGUAGE EN

How to fix this error?

Enter the <GL:partner_function>function</> &V1& into the partner
function screen of the document header. It is then automatically
adopted into the individual items.
Functions AA and AW are a special case, because they can never occur in
the item differently to how they appear in the header, even if this is
set in Customizing.

Error message extract from SAP system. Copyright SAP SE.