Was the 2018 return prepared with TurboTax? We can tell you how to upgrade an already-filed return to PLUS and pay the 19.99 by credit/debit card.
Why are you wanting PLUS? Are you trying to unlock prior returns? If so, have you located the prior returns, and are they in the SAME account that you used for your 2018 return?
We can tell you what to do when you answer all the above.
Be aware there's a free option if a tax return transcript would suit your purpose. You may be able to immediately download a prior-year 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>
