There are 2 methods to delete purchase invoice, See below
NOTE : This operation can not be undo

Method 1:

Step 1: Go to “Purchase Invoice” Menu & click on the down arrow beside the edit option and click on “Delete Invoice” option

Step 2: Click on the “Delete” Button

Method 2:

Step 1: Go to “Purchase Invoice” page & select the checkbox before invoice number & click on the “Delete” button at bottom of page to delete selected invoices.

Step 2: Click on the “Delete” button to verify delete operation.