You do NOT amend a tax return that has been rejected for an error. You correct the error and re-file.
See this TurboTax support FAQ for a rejected tax return - https://ttlc.intuit.com/questions/1901491-how-do-i-fix-a-rejected-return
When you access your online account and land on the Tax Home web page, instead of the page that will Take me to my return, then follow this procedure to get back to the 2018 online tax return.
When you sign onto your online account and land on the Tax Home web page, scroll down and click on Add a state. See screenshot
This will take you back to the 2018 online tax return.