How do I show the product code (SKU) in invoices and requisitions?

The product code is the SKU. To show it in sales invoices: from Templates, click Printable Templates, then Invoices & Estimates, open the template for editing, click Item Columns from the side menu, and select Product Code from the Field list in one of the empty rows, then click Save. For purchase invoices, use the same steps from Purchase Invoices.