When the Opportunity stage is Closed Won, this will check the field (Ready to Create Sales order).
If this checkbox is true (checked), you can click on Create GF Sales Order button to create a Sales Order from the opportunity.
The Items should be linked with Products for Sales orders to add sales lines, you will get the message if it's not linked.
After creating a Sales Order, it will populate on Sales order lookup field on the opportunity.
In the Sales Order, you will have a related opportunity.