the instructions for line 20 of 1040-X would seem to indicate direct debit is not possible
Line 20—Amount You Owe
See the “Amount you owe” section in your income tax return instructions or go to IRS.gov/Payments for payment options.
https://www.irs.gov/payments
you can send a check with the amended return if you mail it or use the IRS Direct Pay
if you e-filed the original you should be able to e-file the amended return (2021 only)
if you mail the amended return use a method where you can track it. you don't get any confirmation that it was received and it can take the IRS 6 to 8 months to get to it.
