You are only able to file using TurboTax Free Edition if you qualify for it.
Please see the following link for tax situations covered in TurboTax Free Edition.
https://ttlc.intuit.com/replies/3301994
If you don't qualify for TurboTax Free Edition, you may qualify for TurboTax Free File Program.
Here is more information on TurboTax Free File Program.
https://ttlc.intuit.com/replies/3301359
You can follow the instructions in the following FAQ to switch to the Free File Program.
https://ttlc.intuit.com/replies/3491786
If you don't qualify for either of the above products, you will be prompted to upgrade to the appropriate product and you will be charged for that product.