No entry permitted in trading partner number field

What causes this issue?

You are not permitted to make an entry in the trading partner field.

System Response

Depending on the context, the system issues either an error or a
warning message. If it is a warning message, the field is deleted.

How to fix this error?

If you want to assign the trading partner to an account, check the
setting in the document type control &V1& (Indicator: Manual entry of
trading partner).
<DS:TRAN.OBA7>Proceed</>

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.