You cannot use TurboTax Online to file for a past year. TurboTax Online is only for the current tax year, which is now 2019. To file for an earlier year you have to use the CD or download TurboTax software for the year that you need. You can purchase a download at the following link. There is no free filing for past years.
Past Years' Products
You might get a better price from a reputable online seller that still has TurboTax in stock for the year that you need.
Tax returns for past years cannot be e-filed. You will have to print your past-year tax return, sign it, and file it by mail. The IRS stopped processing mailed tax returns for several months because of the Covid-19 pandemic. They just recently started to process them again, but they have limited staff and a huge backlog, so it will be quite a while before your past-year tax return is processed.
TurboTax no longer supports years earlier than 2016. It's too late to get a refund for 2016 or earlier. You can file the tax return, but if it shows a refund you will not get the refund. If it shows a payment due, you have to pay it.
