Business partner proposed as payer/payee (see long text)

What causes this issue?

The partner being processed was proposed as the payer/payee. If you
create a new partner with internal number assignment, the system will
enter the value '##1' for the payer/payee, as the partner number is not
yet known. This provisional value is replaced by the actual partner
number when you save the data.

System Response

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

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.