Hello
" So the quotation's status is still to invoice. How can I change the status to "Closed"?"
There might be 2possibilities If your status of sale order is NOT in close/done state:
1. You didnt completed shipment process of sale order.
-> Go to warehosue->delivery order and complete your shipment process.
2 Procurement method of product configuration is make to order.
->If you want to change status of sale order to close/done then 1st u need to check procurement method in your product configuration.
Suppose product ABC whose procurement method ='make to order'->
2.1.When u confirm sale order of ABC product then Purchase quotation automatically generated .
2.2. So in this case you have complete invoicing process and shipment process of PO and your sale order status will be closed/done.