You may have mistakenly added a feature or chose to "upgrade" without knowing. It should tell you on the payment page what exactly you are being charged for.
If you have upgraded from Free to another version of TurboTax, or added a feature such as "Plus" or 'Audit Defense" in error, you cannot complete that return without paying. You will need to create a new return and choose Free.
Once you have started your new return, you can click on "View all tax returns" and click the small Trash Can next to the unwanted return to delete it.