After you file

We can tell you the proper steps to upgrade and pay with credit/debit card.   Have you already done the upgrade part to PLUS but just need to pay?   If you have upgraded and have gone to the Print Center to pay but get sent back to the home page, then you are choosing the wrong option at the Print Center.   We can tell you all the steps.  Whatever you do, do not upgrade any further, or you'll have problems.

Also be aware that you may be able to download a free 2015 tax return transcript at this IRS site.  If you don't qualify for the immediate download, you can order one to come by mail, but that 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>

We can tell you the precise steps to upgrade and pay for PLUS (29.99) with credit/debit card.  First I need to ask some questions:

How to upgrade and pay depends on what you have done for your current 2016 return.    Have you already filed your 2016 return with Online TurboTax, or is it still in preparation?  Was the 2016 return in Free Edition?   Or was it prepared outside of TurboTax?

And is the locked 2015 return in the SAME account as the one you used/are using for the 2016 return?  

We can give you more info when you answer all the questions above.