To file a 2018 tax return using TurboTax, you must buy, download and install on your computer the 2018 program. Here is a link to buy it: https://turbotax.intuit.com/personal-taxes/past-years-products/

Prior year returns can't be efiled. They must be printed and mailed by you.  TurboTax does not print and mail them for you. Look at the filing instructions on the first page of your printed return for what to include and where to mail your return.