To change a return If you haven't mailed or efiled yet, see
https://ttlc.intuit.com/community/tax-topics/help/i-want-to-change-a-return-i-ve-printed-but-haven-t-mailed/00/27616
If you haven't efiled or mailed your return or it rejected you can go back in and change anything you want. Do not click on Amend!
Click on Add a State to let you back into your return.
From Tax Home (the first screen you see after you sign in), scroll down to Your tax returns & documents. You may have to select Show to open the drop down.
Select 2020, and then select Add a State (you're not actually adding a state, this is just the workaround).