Only the 2020 tax return is available to be completed and filed for free if you are eligible.
IRS website for free tax preparation software vendors - https://www.irs.gov/filing/free-file-do-your-federal-taxes-for-free
TurboTax does not support tax year 2016
Prior year tax returns using TurboTax, 2017 thru 2019, you would need to purchase one of the desktop editions for each tax year required 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 using the TurboTax personal editions.
You are too late to get tax refunds for any year earlier than 2018. But if you owe tax due it still has to be paid.
If you requested an extension in 2018 for your 2017 return you can still get a refund for 2017; otherwise you have forfeited tax refunds for 2016 and 2017.
You can prepare past year tax returns on paper and mail them in.
PRIOR YEAR FORMS FROM IRS
Form 1040 Instructions https://www.irs.gov/pub/irs-prior/i1040gi--2016.pdf
Form 1040A https://www.irs.gov/pub/irs-pdf/f1040a.pdf
Form 1040AInstructions https://www.irs.gov/pub/irs-prior/i1040a--2016.pdf
2018 Form 1040, Schedules and Instructions
2018 Tax and EIC Tables https://www.irs.gov/pub/irs-prior/i1040tt--2018.pdf
IRS 2018 Schedules 1 thru 6 (only if needed)
Sch 1 : https://www.irs.gov/pub/irs-prior/f1040s1--2018.pdf
Sch 2 : https://www.irs.gov/pub/irs-prior/f1040s2--2018.pdf
Sch 3 : https://www.irs.gov/pub/irs-prior/f1040s3--2018.pdf
Sch 4 : https://www.irs.gov/pub/irs-prior/f1040s4--2018.pdf
Sch 5 : https://www.irs.gov/pub/irs-prior/f1040s5--2018.pdf
Sch 6 : https://www.irs.gov/pub/irs-prior/f1040s6--2018.pdf
2019 Schedules 1-3
Mail each tax year in its own envelope. Do not combine them in any way.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.
Federal and state returns must be in separate envelopes and they are mailed to different addresses. Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.
Remember to prepare your state returns as well—if you live in a state that has a state income tax.