Good day 

Please note that you unfortunately you cannot remove/delete Invoices, you can do a Customer Credit Note to cancel the effect of invoice. If you process as Customer Adjustment, Journal Entry or Banking transaction, Kindly note that those you can delete from the screen you processed them on.

Please find below the steps to create the Customer Credit Note

Customers… Transactions… Customer Invoice… On the Invoice you want to delete click on action then

The next screen will show the Process Customer Credit Note populated with the same info as the Customer Invoice

You can click on Save

Kind regard

Eliazer