Generally, Odoo doesn't allow you to cancel the purchase order when you confirm the cancelation of the sale order. As a result, the process becomes more complicated as well as time-consuming. 

No need to worry now as we have developed a module that automatically cancels the purchase order when you cancel the sale order. One thing to note here, this module only works for those purchase or dropship orders, which are generated from the sale order. 

The screenshot below shows that you confirmed both the sale and dropship purchase order. When you cancel the sale order, the tailor-made functionality of this module helps you cancel the purchase order automatically. This is the beauty of this application.

After canceling the sale order, it will cancel the dropship purchase order as shown in the screenshot given below.

Now, there is a twist. If a product is delivered once, then this module won't allow you to cancel the sale order as displayed in the screenshot below.

https://apps.odoo.com/apps/modules/14.0/ksc_cancel_po_dropship/