TurboTax has 2 free products - Free Edition and the Free File program. Below are the qualifications to be able to use either of those products. If you have a tax situation outside of these two scenarios, you will need to upgrade to Deluxe to be able to file your taxes through TurboTax. You can check which situations required you to upgrade to Deluxe here:
https://turbotax.intuit.com/personal-taxes/online/deluxe.jsp
For the Free File Program, a user needs to meet only ONE of these qualifications:
AGI of $34,000 or less
or active duty military (including Reservists and National Guard) with a military-issued W-2 and a 2017 AGI of $66,000 or less
or eligible for the Earned Income Credit (EIC aka EITC).
https://turbotax.intuit.com/taxfreedom/
You can use the Free Edition if you have a simple tax situation that uses form 1040 with no attached schedules.