Not sure why you can't set up a new account, up to 5 accounts can be tied to the same email address.

If you still want to start another return in TurboTax Online, here’s what to do:

1. Sign out of TurboTax Online.
2. Select the TurboTax Online product you want to use for the second return and proceed.
3. On the Create your account screen, set up a new user ID for the second return.
4. You can reuse your email address if you want to receive e-file notifications for the second return.
5. Up to 5 accounts can be tied to the same email address.
6. After you've created the new account, you can start working on the second return.
7. If you can't create a new account because your web browser automatically logs you into your old account, you may need to clear your browser history, cache and cookies.