How To Fix BK191 - The external number is not unique, enter an internal number

BK191 - Overview

• Message type: E = Error

• Message class: BK - Create, Display, Reverse Controlling Document

• Message number: 191

• Message text: The external number is not unique, enter an internal number

What causes this issue?

You want to reverse a document. Several Controlling documents were
found for the key.

System Response

If you want to post despite this message, all documents found by the
system will be reversed.

How to fix this error?

If this message is not an error message, check whether you really want
to reverse all documents.
If this message is an error message, you should execute a reversal by
specifying the CO document number and the controlling area.
In Customizing you can either disable this message or change it from an
error message to a warning or a note. To do this, see the
Implementation Guide (IMG) "Controlling General" workstep
<DS:SIMG.SIMG_CFMENUORK3OBA5>"Change message control"</> (Work area
"BK", Message number "191").

Error message extract from SAP system. Copyright SAP SE.

Smart SAP Assistant

• What is the cause and solution for SAP error message BK191 - The external number is not unique, enter an internal number ?

  The SAP error message BK191, which states "The external number is not unique, enter an internal number," typically occurs in the context of financial document postings, particularly when dealing with external number ranges for document numbers. This error indicates that the system has detected a duplicate external document number that is already in use, and therefore, it cannot accept the new entry.
  
  Cause:
  
  Duplicate External Number: The external document number you are trying to enter already exists in the system. SAP requires that external document numbers be unique to avoid confusion and ensure data integrity.
  Configuration Settings: The number range settings for the document type may be configured to use external numbers, and the system is enforcing uniqueness.
  Incorrect Number Range Assignment: The document type may be incorrectly assigned to a number range that is not set up properly.
  
  
  Solution:
  
  Check Existing Document Numbers: Verify if the external document number you are trying to use is