How to cancel pending transactions on Apple Account?

It may depend upon under what general category you made your purchase.

For Apple Store purchase see Viewing & Changing Orders - Shopping Help - Apple

If you mean an app or media purchase, you cannot. You have to wait for them to clear and then ask fora refund (which is not guaranteed). Click on this link to read how to 'Request a refund for apps or content that you bought from Apple' -->

Request a refund for apps or content that you bought from Apple - Apple Support

"If the charge is pending, you canʼt request a refund yet. After the charge goes through, try to request a refund again."