How To Fix WY123 - Customer & is already assigned to another supplier

WY123 - Overview

• Message type: E = Error

• Message class: WY - Supplier Master

• Message number: 123

• Message text: Customer & is already assigned to another supplier

What causes this issue?

You wanted to assign a customer to the vendor. However, this customer
is already assigned to another vendor. You cannot carry out the
function.

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?

Please enter a different customer.

Error message extract from SAP system. Copyright SAP SE.

WY123 - Details

• SAP Error Message:
  WY123 Customer & is already assigned to another supplier
  
  
  Cause:
  This error occurs when you try to assign a customer master record to a supplier master record, but the customer is already linked or assigned to a different supplier in the system. In SAP, a customer can only be assigned to one supplier at a time in certain business contexts (e.g., in vendor-customer integration scenarios, or when using the Customer-Vendor Integration (CVI) in S/4HANA).
  
  Explanation:
  
  The system prevents duplicate or conflicting assignments to maintain data consistency.
  This typically happens during master data synchronization or when creating/updating business partner relationships.
  For example, in SAP S/4HANA, the Business Partner (BP) approach integrates customer and vendor data. If a customer is already linked to a supplier BP, trying to assign it again to another supplier BP triggers this error.
  
  
  
  Solution:
  
  
  Check Existing Assignments:
  
  Identify which supplier the customer is currently assigned to.
  Use transaction codes like BP (Business Partner) or XK03