An order type to send free samples to the customer has been developed, ie ZFRS. The item category used is KLN. Creation of the invoice is allowed before goods issue. The client now wants that GI be made mandatory before billing. I tried changing the requirement to 003 at heade level and 004 at item level in copying control from the delivery to the billing document. This is used in copying control from LF to F2. However, now while trying to create a billing document, an error is coming 'Doc type is not relevant for billing'. What should I do to make GI mandatory before creation of the proforma invoice?