When you try to view/print/download your 2016 return, does it tell you that you need to upgrade to PLUS and pay to get access? And have you already done the upgrading part? I can tell you how to pay for your upgrade.
Also be aware that if a tax return transcript will suit your purpose, you may be able to download a free tax return transcript at this IRS website. If you can't download it, then you could order it to come by postal mail, but that takes 5-10 calendar days.
Since you said you have already upgraded your 2017 return to PLUS, I will skip the upgrade steps and provide the payment steps.
- Log in and open the 2017 return you started and already upgraded "Take me to my return."
- Once the return is open and past the blue-green screen, click in the left menu column on Tax Tools, then choose Print Center.
Then choose "Print/save/preview this year's return." (Yes, you say "this year's" since you are paying for the 2017 return to unlock the prior returns.) You won't actually have to print it.
- On the next screen choose to "Pay now", and it should then prompt you for payment by credit or debit card.