Enabling bill button in sales order and item fulfillment record:
Enable “Invoice in Advance of Fulfillment” in Accounting Preference to enable Bill button in sales order and item fulfillment record.
For that Navigate to SetUp> Accounting > Accounting Preference > Order Management, tick the checkbox, “Invoice in Advance of Fulfillment“
Bill button will be visible in item fulfillment and sales order record,
Note: If the quantity invoiced is greater than the quantity fulfilled on the Sales Order, button will not be visible on Item Fulfillment record