How come I can't start over if I hadn't filed yet for?

Here's how you can start over in either the same or a new Online product you choose, as long as you haven't paid for TurboTax Online.

NOTE: Starting over clears all of your current entries. All entered information will have to be re-entered in the new Online product you start over in.

1. While in your TurboTax Online program (after clicking Continue your return)
   Select My Account, then click on Clear & start over.
2. On the Are you sure you want to clear your return? pop up screen, if you are sure about clearing all your entries and starting over, click Yes. 
   Then a new screen confirms you have cleared your entries, click Continue.
3. A Help us find the right TurboTax product for you screen shows the TurboTax Online products and features.
4. Select your new TurboTax Online product by clicking the Start for Free button.

Once your new TurboTax Online product is selected, you can return to your taxes.

https://ttlc.intuit.com/questions/2586254-how-do-i-clear-and-start-over-in-turbotax-online

How do I clear and start over in TurboTax Online?