I wish you had asked this question several weeks ago to avoid the frustration. We can tell you a special way to pay. I need to ask a few questions first.
1) Was the 2016 return prepared online, and have you located it in your account?
2) When you try to download/print/view the 2016 return, does it tell you that you need to pay?
3) Have you already filed a 2017 online return, or is the online 2017 return still in preparation?
4) What edition are you currently in for 2017? By your screen name above it says "Free Edition", yet you mentioned above that you had upgraded to PLUS, and then later upgraded again to Deluxe.
5) Is the 2016 return in the SAME account that you are using for your 2017 return?
Be aware there's a free option if a tax return transcript would suit your purpose. You may be able to immediately download a 2016 return transcript at this IRS website at the link below. If you don't qualify to open a free IRS account there, you can order one to come by postal mail, which takes 5-10 calendar days.
<a rel="nofollow" target="_blank" href="
http://www.irs.gov/Individuals/Get-Transcript">http://www.irs.gov/Individuals/Get-Transcript</a>