Ledger &1 is an extension ledger (contr. area &2, version &3)

What causes this issue?

You have assigned ledger &V1& to controlling area &V2& and CO version
&V3& in customizing activity <DS:SIMG.FINSV_VERSN_LD>Define Ledger for
CO Version</>.

System Response

Ledger &V1& has ledger type <LS>Extension Ledger</>. You can't assign an
extension ledger here.

How to fix this error?

Please enter a ledger with ledger type <LS>Standard Ledger</>.
You can check the ledger type using
<DS:TRAN.FINSC_LEDGER>transaction FINSC_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.