You cannot cancel or change anything while your return is in pending. What payment are you trying to cancel?
It is not possible to cancel a payment when the tax return is Pending. You can cancel the payment after the IRS receives the tax return.
Go to this IRS website for how to cancel a payment - https://www.irs.gov/payments/pay-taxes-by-electronic-funds-withdrawal
Once the return has processed, you can follow the steps provided by DoninGA above. I have included her response here for easy review:
"Go to this IRS website for how to cancel a payment - https://www.irs.gov/payments/pay-taxes-by-electronic-funds-withdrawal