I need to gain access to my tax return from 2017 and followed the instructions to upgrade, but realized I have the Deluxe version already. Why can I not access my returns

Then you only need to pay for the 2018 Deluxe edition you are using to have access to all prior year online returns, provided you created the 2018 account using the exact same User ID that you used to create the prior year online accounts.

To pay the TurboTax online account fees by credit card so you can access your prior year online tax returns, click on Tax Tools on the left side of the online program screen.  Then click on Print Center.  Then click on Print, save or preview this year's return.  On the next page, to pay by credit card, click Pay Now.  On the next screen just click on the Continue button.  The next screen will show a review of your Fees, click on Continue.  Complete the next screen to pay by credit card and click Continue.