If you came across GL Sets of books error as soon as you try to navigate from Order Header to Order Lines , make sure that you have configured the System > System Options in Oracle Receivable for your Operating Unit.
Last week I was involved in Complete Order to GL Setup in a Blank R12 Instance and had hard time to resolve this Error. During debugging I notice that whenever you navigate from Order Header to Order Lines Block , System try to validate if for a operating unit if data is available in AR_SYSTEM_PARAMETERS table or not . If there is no data exists in this table for the operating Unit where you are trying to define the Sales order system was throwing Error 'GL Sets of Books" not found ( user-defined error ) , even though General Ledger set up proper
To resolve the issue I navigate to Receivable > Setup > System > System options and define the System options for the Operating Units.