can customer cancel their savings plan?

As CloudJason mentioned, the normal policy is no. You cannot cancel a SavingsPlan if you've purchased as they are a contractual agreement between you and AWS for a discount on services. But if it was purchased in error, you can request a cancellation but it's not guaranteed to be approved.

If it's rather soon after purchasing, and the first time you've done this, you can contact support and they will usually cancel it, usually a day or two. If it was a mistake there are times support will escalate and cancel it although they don't often like to do that as it is a contract and they don't want people backing out of them.

I made a mistake when calculating the amount of my saving plan, I changed the amount in hours for the amount per month, I immediately noticed the error but I was not able to cancel. My plan that was supposed to be around 40USD/month went to 29k USD/month