Special G/L ind. & for b/exch.request must be classified as b/exch.

What causes this issue?

In the system configuration, a special G/L indicator which is not
classified as a bill of exchange was specified for the posting of bill
of exchange payment requests.

How to fix this error?

If the correct special G/L indicator was allocated, correct the
classification of the indicator in FI Customizing. Otherwise, allocate
a different special G/L indicator to the transaction 'Bill of exchange
payment request' in Customizing.

Error message extract from SAP system. Copyright SAP SE.