Isolta Online service is really quick sending invoices so the invoice you just sent can't be cancelled. If you found a mistake in the invoice, we recommend you to contact your client as soon as possible. 

You have couple of options what to do next. You can create a credit note and send it to your customer. After that you can create a new invoice. This is the recommended way if you have sent an e-invoice. 

Another option is to modify the invoice and send it again to your customer.