If you haven't efiled or mailed your return you can go back and switch to efile. 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 Add a State (you're not actually adding a state, this is just the workaround).
Then when you are ready to file you can try to switch to efile.
https://ttlc.intuit.com/community/e-file/help/how-do-i-change-from-mail-to-e-file-in-turbotax-online/00/26985