We can tell you all the steps to pay to unlock prior returns with better instructions than TurboTax does. But you have a problem to resolve first if you have upgraded too far to Deluxe. The minimum fee is to upgrade to PLUS at 19.99, but since you upgraded too far, if you try to pay now, it will charge you 59.99 for Federal and 44.99 for a state return. We can tell you what to do about that.
When you tried to download your 2017 return, did it tell you that you have to pay to unlock access? Please answer all of the following:
We can tell you how to unlock a return once you answer all the questions 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.