First, be sure the 2017 and 2018 returns are in the same account.
It sounds like you may have upgraded the 2018 return, but then didn't pay the 19.99 by credit/debit card. It is the paying that unlocks the prior returns.
If you have upgraded your 2018 return to PLUS, then here's how to pay for it. Whatever you do, do not upgrade any further, or you'll have problems.
Open your 2018 return, if not already open.
If you are sure you are upgraded to PLUS, then go to the left menu column, click on TAX TOOLS, then choose PRINT CENTER.
At the Print Center choose to "Print, save, view THIS YEAR's return." (Yes, you say "this year's" because you are paying for the 2018 return upgrade in order to unlock the 2017 return.) You don't actually have to print the 2018 return.
It will then ask you to pay. If the Order Summary appears OK (PLUS at 19.99), then choose PAY NOW.
You can pay by credit card or debit card (even though it only says "credit.") If it should offer you the choice of paying later out of the Federal refund, that cannot be done for this purpose. You have to choose PAY NOW.
Once paid, then go back to the section "Your Tax Returns & Documents", choose 2017 tax year, and it should let you download/print the 2017 return.
If you have actually paid the 19.99 by credit/debit card and the charge went through, but it's still not letting you download the 2017 return, report back.