Can I file previous year taxes electronically and FREE?
No you cannot.
Using TurboTax for prior year tax returns, 2016 thru 2018, you would have to be purchased download and install on a personal computer, Win or Mac, one of the prior year desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
Prior year tax returns can only be printed and mailed, they cannot be e-filed.
You can get the forms directly from the feds and state, fill them in manually and mail them in for the cost of paper, ink and a stamp. All prior year returns must be mailed in anyway even if you use a program.
Interesting ... the website is switching for the current year forms. Here is the link to all year forms ... make sure to pick the 2018 ones...