Project must be used for production company codes

What causes this issue?

You are assigning a new ledger to a production company code without
using a project for the subsequent implementation of a ledger.

System Response

The assignment is not made.

How to fix this error?

To ensure data consistency in a company code that is already in use, you
first need to perform the complete maintenance of a project. This
contains the following steps:
Create a project for the introduction of an accounting principle. Follow
the steps in the Customizing activity <DS:SIMG.FINSVC_SIL_PROJ>Create
Project for Implementing Accounting Principle</>
As part of the project definition, define the new ledger (target
ledger).
Assign the relevant company codes and the source ledger to the project
and target ledger.
Since the company code to which the ledger is assigned is already in
productive use, you need to use the project to transfer the data of
source ledgers. Otherwise, a consistent dataset cannot be guaranteed in
the new ledger.

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.