If you haven't efiled or mailed your return 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).
Maybe you should consider using the Desktop program. Then you can see all the forms and enter directly into them. And the Desktop program has more features and other advantages over the Online version.
