Once an Invoice has been assigned an Invoice Number (field: number) you will get the error "You cannot delete an invoice after it has been validated (and received a number). You can set it back to "Draft" state and modify its content, then re-confirm it.".

To enable Invoices to be deleted you need to both allow cancelling and make this field visible and editable.

Note you are bypassing business logic that was put in place to prevent the deleting of documents that, by law in some countries, is not permitted.  Also, you will end up with gaps in your Invoice sequence (a red flag for auditors).