No entry in FAGLFLEXA or FAGLFLEXT for ledger &2 and company code &1

What causes this issue?

For ledger &V2& and company code &V1& there are no entries in tables
FAGLFLEXA or FAGLFLEXT. You can check this in the current system in the
views FGLV_GLSI_SRC and FGLV_GLTT_SRC.
If company code &V1& was a classic general ledger company code in the
system from which you are migrating the data and you have decided not to
use it in the new general ledger, then you can proceed with the
migration.
If this is not the case and FAGLFLEXA or FAGLFLEXT were previously
filled in the system from which you are migrating the data, you must
check the data of FAGLFLEXA or FAGLFLEXT and determine why it does not
exist in the new system.

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.