You do NOT have to "refile" a 2018 return to unlock a past return. In fact, it is not possible to efile twice anyway in the same account. The TurboTax instructions are indeed confusing. Perhaps you read to pay in the File area (without actually filing), but in this forum, we can normally tell you alternate and better steps to get to a dedicated payment area at the Print Center. However, you may have an additional problem to overcome.
Is access to your 2017 return locked, and it's asking you to pay? And have you already filed the 2018 return? If so, did you accidentally upgrade too far to Deluxe (that's what is showing underneath your question above?) If you accidentally upgraded to Deluxe after filing your return with Online TurboTax, then if you try to pay now, it will charge you 59.99 for a Federal return, and 44.99 for a State return, if applicable.
The minimum to unlock a 2017 return is to upgrade a 2018 Free Edition return to PLUS and pay 19.99 by credit/debit card using special steps we can tell you. However, since you have already filed, you won't be able to downgrade it yourself.
Or did you file your 2018 return outside of TurboTax? If you didn't use TurboTax for the 2018 return, then you can downgrade from Deluxe. I'll mention more in a following comment.
But if you upgraded an already-filed 2018 TurboTax return too far, here's what to do:
If all of the above describes your dilemma, you'll need to phone TurboTax Support during the day, tell them you upgraded too far accidentally after filing your 2018 return, and ask them to just unlock the 2017 return. They should do that for you and may not even charge you.
Hours are 5AM-5PM Pacific (8AM-8PM Eastern) Monday-Friday.
FAQ: What is the TurboTax phone number?
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.
How did you prepare your original 2018 return you filed? Did you use Online TurboTax, and then upgraded too far to Deluxe after filing? If so, then you will need to phone TurboTax Support as discussed above.
Or did you prepare your 2018 return outside of Online TurboTax--either using the desktop software or another provider? If that's your situation, you can still salvage the situation. We can tell you all the steps on what to do if that is your situation, once you clarify.