The possible reasons are,
The absence of the Fulfill button on a sales order can be attributed to several factors. Firstly, it may be due to the sales order being in a Pending Fulfillment status, as indicated in the documents for NetSuite 2019.2, 2022.1, and 2022.2. In such cases, the Fulfill button is not displayed.123
Another reason could be the Payment Hold status of the sales order, as mentioned in the document for NetSuite 2020.2. Sales orders on hold cannot be fulfilled, and this status may not always be visibly displayed when using custom sales order forms.3
Additionally, the Fulfill button may be missing if the Role assigned to the User lacks the necessary Item Fulfillment permission, as suggested in the document for NetSuite 2022.1.2
Furthermore, the absence of the Fulfill button could be related to the Multi-Location Inventory feature being enabled, as described in the document for NetSuite 2022.1. If the sales order is in ‘Pending Fulfillment’ status and has no associated location set, the Fulfill button will not appear.4
Lastly, for a Special Order Inventory Item, the Fulfill button is available even when the item has not been received yet, as stated in the document for NetSuite 2021.2.6