"says I am using PLUS already when I have not."
When you open your 2018 return and go to the Upgrade area, if the PLUS option is grayed out, then you are already in PLUS. If that's your situation, then you just need to skip to the payment steps at the Print Center as described in those articles above, and pay the 19.99 by credit/debit card. It is the paying that unlocks the prior returns.
Whatever you do, do not upgrade any farther than PLUS, or you'll have problems.
There's also a free option if a return transcript will suit your purpose. You may be able to immediately download one at the IRS website if you qualify to open an IRS account there. Otherwise, to order a transcript to come by postal mail takes 5-10 calendar days.
http://www.irs.gov/Individuals/Get-Transcript