BAdI is internal SAP; implementation in customer namespace not allowed

What causes this issue?

The BAdi to be implemented is declared as "SAP internal". No customer
implementation may be created for this BAdI.
If you wish to create an SAP advance delivery of the BAdI
implementation, create this in an enhancement object in the SAP
namespace.

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.