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.
Ok ... there is a DIFFERENCE between starting a NEW file and TRANSFERRING in the 2017 info and OPENING a 2018 tax file.
If you have the .tax2018 file on your computer and you need to open it to complete the return then download/install/update the 2018 program and click on FILE in the upper left corner then OPEN and browse for the file on your computer.
ADDENDUM:
If your 2018 tax data file has a state return in it, then you will need to download the state software into your TurboTax program first before opening the tax data file, or it will give you an error notice.
If it gives you such an error, you may have to minimally start a new fake 2018 return in the 2018 software to download the state program. You can delete that fake return later. Be sure you delete the right one and not your actual tax data file. So be sure to name it something like "fake" or "test" or something so you'll know.