This feature is turned off by default, so you need to enable it in the Company Setting.
Step 1: Go to Basic Settings > App Settings and scroll down.
Step 2: Turn on this feature in Sales Order Settings and Customer Invoice Settings.
Step 3: Remember to save your setting in the upper right corner.
The Result
In the Sales Order (Edit mode and after issuing DOs)
In the Delivery Order, the shipment PDF
In Customer Invoice
