Let's back up. You need to cancel an amended 2018 return? Please e plain that. What have you done and actually filed? Did you mail a 2018 1040X Amended return? Maybe you don't need to do anything for 2018.
You can only amend a 2018 Online return by switching to the Desktop program.
https://ttlc.intuit.com/community/tax-topics/help/how-do-i-amend-my-2018-turbotax-online-return/00/9...
