How can I just file my state tax return if I already filed my federal return?

Sorry you can only efile state if the federal was efiled and accepted from the same Turbo Tax account.  You have to print and mail state.  And pay for both federal and state.  

How to mail a return from the Online version

https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online/00/26258

The state tax return is based on the federal tax return. In order to prepare a state tax return in TurboTax, you first have to prepare your federal tax return in TurboTax, even if you won't file it with TurboTax. Since you already filed a federal tax return somewhere else, you have to make sure that the federal tax return you create in TurboTax exactly matches the one that you filed. Also, you will not be able to e-file your state tax return with TurboTax if you didn't e-file your federal tax return with TurboTax. You would have to print the state tax return and file it by mail.

Obviously it's much simpler and easier to file your federal and state tax returns in the same place. In the future I suggest that you use a company or service that will do both the federal and state tax returns.