I don't think you're clicking on the right thing at the Print Center and so you ended up in a loop. I'll give you the complete steps on how to properly pay, once I verify some things.
When you try to download/print your 2017 return, is access locked, and it asks you to pay? And are the 2017 and 2018 returns in the SAME account?
Have you already filed the 2018 return, or is it still in preparation?
I'll give you all the steps on how to properly pay for your PLUS upgrade when you answer all those questions.
Be aware there's a free option if a tax return transcript would suit your purpose. You may be able to immediately download a 2017 return transcript at the 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 normally 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>
