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.