The best thing to do in this case is to contact your tutor. :)

If the lesson is confirmed, you cannot cancel before. If you inform your tutor in advance, he/she will probably agree to state that the lesson did not occur and return your credits to you. This can be done after the lesson. Or you can email Support at italki to ask them to cancel before the lesson. Again, the tutor must agree.

Are your lessons booked on italki.

I would send a message to the tutor.