Exch. rate differences not allowed for clearing provisional postings

What causes this issue?

Exchange rate differences arose during clearing of provisional postings.
The cause is one of the following:
Exchange rate differences are only allowed for postings without a ledger
group; however, provisional postings with a ledger group are being
cleared.
Exchange rate differences are only allowed for postings with a ledger
group; however, provisional postings without a ledger group are being
cleared.

System Response

The document cannot be posted.

How to fix this error?

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

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.