You cannot use TurboTax to prepare tax returns for clients. TurboTax is only for consumer use and cannot be used by paid preparers. Intuit (the company that makes TurboTax) has three tax software product lines for professional preparers: ProSeries, Lacerte, and ProConnect Tax Online. You can find information about all three product lines at the following link.
https://proconnect.intuit.com/
TurboTax Online cannot be used to file for past years. To file for past years you have to use the CD or download TurboTax software for the years that you need. You can purchase downloads at the following link. TurboTax is no longer available for years earlier than 2015.
http://turbotax.intuit.com/personal-taxes/past-years-products.jsp
