What happens when you click on Amend for 2018? I think there is a new thing where you can amend a 2018 return online. What steps did it give you? Here is the amend article but it just says to follow the steps.
https://ttlc.intuit.com/community/tax-topics/help/how-do-i-amend-my-2018-turbotax-online-return/00/9...
