Ledger group &1 contains extension ledger &2 and underlying ledger &3

What causes this issue?

You have created ledger group &V1& which contains extension ledger &V2&
and underlying ledger &V3& of this extension ledger.
The data of the underlying ledger, however, is valid for the extension
ledger anyway, which is why this would lead to a duplication of the data
in the extension ledger.

System Response

The system does not allow postings in this ledger group.

How to fix this error?

Go to Customizing activity
<DS:SIMG.VC_FAGL_LDGRPMAP_DC>Define Ledger Group</> and remove
extension ledger &V2& from ledger group &V1&.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

You can create ledger groups with one or more extension or standard
ledgers. But if you do so, make sure that no underlying ledger of an
extension ledger is contained in the same group.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.