You can Cancel PO directly by "Cancel" button. If Purchase Shipment is already in "Done" state then you can not Cancel PO.

But however you can apply forcefully Database query to achieve this.

Hi Niyas,

I've referred to the link you sent, I've also tried to return the products but the status of the purchase order is still "Waiting Bill".

When I try to cancel the PO, it says "Unable to cancel purchase order PO00008 as some receptions have already been done.".  
How do you cancel the receptions?

Thank you.

Hi Camille,

To get the details you can refer this Odoo documentation regarding the canceling of purchase order.

https://www.odoo.com/documentation/user/9.0/purchase/purchases/rfq/cancel.html

Also you can have a look at this community app, Purchase Order Cancel