Bill(s) can only be created after the purchase order is confirmed.

To start bill the purchase, go to the bills tab in the purchase or click Bills.

Two ways to bill: 

To partially bill the purchase, select parts of the purchase order to bill e.g. some line items only or lesser quantity etc and then click Confirm.

To completely bill the purchase, just click Confirm and it will automatically creates a bill with all the unbilled items in this purchase order. 

If you wish to edit the bill you created, simply find the bill and click on 'Edit' to make any changes to the bill and click save.