Existing BP &1 will be assigned; maintain customer and supplier

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 currently neither the
required customer nor the required supplier assigned.

System Response

It is not possible to assign a business partner without a customer to a
plant. If the plant profile of the plant also requires a supplier, it is
not possible to assign a business partner without a supplier to the
plant.

How to fix this error?

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

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.