Error while processing customer &1; cannot change retail site assignment

What causes this issue?

You have started ALE inbound processing of an IDoc for a customer that
is assigned to a retail site in the target system.
The IDoc field E1KNA1M-WERKS is filled with a different value than the
value in the target system.

System Response

Changing the plant assignment of a customer that is assigned to a retail
site is not supported. The system stops processing the IDoc.

How to fix this error?

Adapt the data in the source system accordingly.

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.