When trying to process Billing Request I get this message:

"The Sales Invoice Document Number field is required in Invoice-Source Relations."

When trying to process Billing Request I get this message:

"The Sales Invoice Document Number field is required in Invoice-Source Relations."

Did you got to set up the invoice transaction type and Series Number for invoice in the Order types ?

filling invoice doc no on sales order type applicable on baan 4.
On ERP LN, you should fill in on SLI parameter