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.
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.