Postings already exist

What causes this issue?

If you change these customizing settings, the data in the current system
will become inconsistent.

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?

You can however consider a change in the customizing settings in the
following cases:
You are currently working in a customizing or test system and you are
sure that in the production system there haven't been any postings made
yet.
You are willing to accept the inconsistencies in the current system.
In this case you can switch off this message or turn it into a warning
message.
INCLUDE CUSTOMIZED_MESSAGE_INFO OBJECT DOKU ID TX

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.