If you've paid the 19.99 for the 2018 PLUS upgrade with a debit card in the same account as the 2017 return (and you can see that the charge was approved and the debit occurred in your bank account), and the 2017 return is not unlocking even after the browser maneuvers discussed above, you'll need to phone TurboTax Support and let them troubleshoot it with you, and ask them to unlock the past return.
FAQ: What is the TurboTax phone number?
https://ttlc.intuit.com/replies/3300041
