Item Validation Organization:

Navigation: Order Management > Setup > System Parameters > Values

•  Item Validation Organization system parameter is the key parameter which should be defined with out which items cannot be placed on Sales orders.
• While entering Sales Orders, Order Management validates if the item is orderable in the organization specified as Item Validation Organization. It does not validate against the organization / warehouse specified on Sales Order.
• Only items that have "Customer Orders Enabled" item attribute set to Yes in Item Validation Organization will appear in the item LOV on sales order line.