CO and FI-GL postings are integrated by design in S/4HANA

What causes this issue?

CO and FI-GL postings are integrated by design in S/4HANA:
Reconciliation posting/real-time integration CO-FI is obsolete.

System Response

In S/4HANA Accounting and Controlling are based on the same table ACDOCA
(instead of separate tables like COEP and FAGLFLEXA/BSEG). It is
therefore not possible to change the G/L accounts in FI only: The
accounts in Controlling and Accounting documents are per definition
identical.

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.