You can close the sales order in two different ways:
1. Close the line items of the order.
Closing all the line items of the order will also close the sales order.
2. Customize the form to show the Close Button.
Try to check if the Close Button Can displayed on the sales order by:
View a sales order
Click on Customize > Customize Form , located at the top right of the screen
Go to the Actions tab
Under Standard Actions look for CLOSE and make sure that the SHOW checkbox is checked.