Partner account assignment cannot be found for item &1 in method &2

What causes this issue?

The posting indicator in Customizing is set to "Post to partner unit".
The partner assignment of the triggering item cannot be determined.
There can be two reasons for this:
Triggering item &V1& does not support a partner assignment.
In the case of reclassification triggered by the debit/credit sign, the
indicator "Balance per partner unit" is not set.

How to fix this error?

Deactivate the indicator "Post to partner unit" in method &V2& in
Customizing, or set the indicator "Balance per partner unit" if
reclassification is triggered by the debit/credit sign.

Error message extract from SAP system. Copyright SAP SE.