Ledger & is still using a pool table; see long text

What causes this issue?

You are attempting to post to an FI-SL pooled table. However, FI-SL
pooled tables are not to be used since Release 30F because they have
significant disadvantages compared to transparent tables (in terms of
performance and restricted functions, for example).

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?

Check whether you really use the ledger with the pooled table. Often,
the ledger L0 for pooled table GLT1 delivered by SAP is updated without
actually being used or valuated. Bear in mind that updating this type of
ledger unnecessarily costs both performance and memory space.
If the ledger is no longer required, deactivate it with Customizing
transaction GCL2. To do so, select the ledger and first delete all
activity assignments (both assignments directly to the ledger and
assignments to the ledger/company code or ledger/company combination).
Then deactivate the 'Ledger Posting Allowed' indicator in 'Additional
Settings' on the ledger master data screen. From now on, the ledger will
not be posted.
If the ledger for the pooled table is still required, convert to a
transparent table as soon as possible by implementing SAP Note 1014076.
In the exceptional case that you still want to use the ledger with the
pooled table and no longer want to see this information message when
posting, you can deactivate message GI128 with transaction OBA5 by
making a corresponding entry in table T100C.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.