Sales Invoice is created from Delivery Order. Please refer to the delivery order guide for help.
If you just created a delivery order, you may click the Create Credit Invoice button on the delivery order view page to open the sales invoice screen.
Otherwise, you may open the Delivery Order List to find your delivery order. There is a Create Credit Invoice link at the right side for each delivery order.
Please note, if the delivery order has been converted to sales invoice completely, the Create Credit Invoice link will be disabled.