Close Order button will not be available anymore when:
1) The Status of the Sales Order is Pending Approval, Pending Fulfillment, Billed, and Closed.
*A sales order needs to be approved first before it can be closed. This is logical since an unapproved sales order is still not a valid transaction.
*Once a Sales Order has been fulfilled and invoiced (Billed) it implies that the company do not have the intention to close the order anymore.
2) The Custom form used for Sales Orders is customized to hide the Close Order button.
*To customize the Sales Order form to show the Close Order button, check Answer ID: <to be populated> Customize Sales Order form to show Close Order Button.