Command file not defined

What causes this issue?

The name of the command file has not been given in the port
maintenance.

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?

Maintain the command file using the transaction <DS:TRAN.WE21>Ports in
IDoc Processing</>

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.