Existing BP &1 with supplier &2 will be assigned; maintain customer

What causes this issue?

You are creating a plant and need to assign a business partner to the
plant.
The business partner you want to assign has the required supplier
maintained, but currently no customer is assigned.

System Response

It is not possible to assign a business partner without a customer to a
plant.

How to fix this error?

Use the business partner maintenance to extend the business partner by a
customer.

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.