Why doesn’t the “Mark as Delivered” option appear on the sales invoice?

Because the Requisitions for Sales Invoices setting is enabled in Inventory Settings. When it’s enabled, the system hides everything related to delivery from the invoice, and the requisition flow continues to work as usual:

• On the invoice creation screen: The Mark as Delivered option and the Delivery Date field don’t appear.
• On the saved invoice: The delivery status (Delivered / Undelivered) doesn’t appear next to the payment status, and neither do the Mark as Delivered and Mark as Undelivered options, whether in the ⋯ menu or in the Stock tab.
• On save: The system creates the requisition automatically as usual.

To show the delivery options, disable Requisitions for Sales Invoices from Inventory Settings.

Note: If the setting is disabled and the option still doesn’t appear, check where you’re looking. When creating the invoice, you’ll find it in the Delivery tab. On a saved invoice, you’ll find it in the More ⋯ menu in the actions bar. In the Stock tab, it appears next to an item when you hover over it.