You're posting in someone else's unanswered thread who happens to be in the Premier Edition, so your situation may not be exactly the same. It might get confusing helping two folks in one thread, but I can help you with your situation once it becomes clear. Please answer all questions:
- Are you wanting to pay for 2018 PLUS to unlock prior returns that are in the same account?
- What is the story on your 2018 return? Has it already been filed, or is it still in preparation?
- What edition are you in right now? Have you already upgraded your 2018 Free Edition to PLUS, and now just need to pay for it? If the PLUS selection is grayed out, that means you already have it. Do not upgrade any farther, or you'll have problems. If you are in PLUS, you just need to pay.
- Is PLUS the only thing grayed out? If anything else is grayed out, then you are in an edition higher than PLUS.
I can tell you the steps to pay the 19.99 by credit/debit card, when you answer all of the above.
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.