If you did not file in 2016 you can do so by going to Filing Past Tax Returns and following the instructions. However, you do not have to file 2016 to file 2017. If you did not file in 2016 you will put a zero when asked for your AGI as you did not have one for 2016. I would recommend that you do file your 2016 as soon as you can to prevent penalties and interest increasing if you will owe, or so you can get your refund before the statute of limitation (3 years) runs out and you cannot collect it.
To learn more about AGI and its uses for 2017 tax return, please read the TurboTax FAQ below. Information for 2016 non-files is in the first 2 lines.
https://ttlc.intuit.com/replies/3301618