One of the splitting methods delivered by SAP is active; see long text

What causes this issue?

You want to create a new document splitting rule but you use splitting
method &V1& delivered by SAP.

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?

We recommend that you copy the delivered default settings to the
splitting method that you have defined and then make changes to the
entries. Proceed as follows:
Select the entries of the splitting method template, choose <LS>Copy
as...</>, and then specify your own splitting method.
In the dialog box that appears, confirm by choosing <LS>Copy [All
Dependent] Entries</>.
Adjust the copied splitting rules according to your needs.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.