If the program didn't register the fact that you mailed in the original return then go to the FILE tab and choose to mail in the state return ... then close the program and reopen it to register the filing. Then you should be able to amend the fed and/or state without issue.
See this TurboTax FAQ for help with amending:
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
