You cannot change the supplier of a retail site customer

What causes this issue?

You have tried to change the value of the <LS>Supplier</> field in the
customer master data of a business partner that is assigned to a retail
site.

System Response

If a business partner is assigned to a retail site, it is not possible
to change any assignments to a customer, a plant, or a supplier.

How to fix this error?

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

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.