The only time 2018 TurboTax asks for a 2017 tax data file, is if one is starting a new 2018 return and it asks you if you want to transfer the 2017 data to 2018.
If you have 2018 software installed and have a copy of your already-started *.tax2018 data file, open it this way:
Launch TurboTax.
At the main screen, go to the top left corner of the TurboTax interface, and click on the FILE tab. In the dropdown menu choose "open tax return" and point TurboTax to the location of the 2018 tax data file.
