System is showing credit message at VAO1 because of configuration in OVAD at the time of sales order creation and there is some overdue balance for customer which can be seen in FBL5N. Ideally you should have this check at DO Creation because sale order is a memorandum document. This is for your information.
Its very easy . Run customer in FBL5N , at open item criteria, if there are some invoices with overdue status it means customer has overdue balance. The ideal thing here is to create new customer and then do the testing if you are in development server.
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