See the info below on how to contact TurboTax Support to inform them of your 2018 software purchase. That qualifies to unlock prior online returns, but you'll have to ask them to do it.
I presume you also bought the 2017 desktop software (CD/download) since you need it to prepare the 2017 return. Once TurboTax Support unlocks your prior online returns, you can download the *.tax2016 data file and transfer it into a new 2017 desktop return. The 2017 return cannot be efiled. It has to be printed, signed, dated, and mailed. If you have both Federal and state returns, you mail them separately to the respective tax agencies.
Once you finish the 2017 return, you can transfer the *.tax2017 data file into a new 2018 return in the 2018 software.
