I'm using iOS 9.3.5 and can't upgrade

@Joekrumm48 

I added some additional info to this answer including an additional FAQ concerning the Mobile App in case that's what you are referring to.

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Exactly "how" are you trying to prepare your return?   Are you using Online TurboTax with a browser on a mobile device or the Mobile App?

In this minimum system requirements for Online TurboTax (browser version) it shows iOS 9.x under "mobile/other" (the last blue subtopic).

FAQ: TurboTax Online system requirements

https://ttlc.intuit.com/community/system-requirements/help/turbotax-online-system-requirements/01/25787

While you can use Online TurboTax through a browser, you can't use the Mobile App, however, which requires iOS 10 and above.  See this:

FAQ:  What are the system requirements for the TurboTax mobile app?

https://ttlc.intuit.com/community/system-requirements/help/what-are-the-system-requirements-for-the-turbotax-mobile-app/01/26757