The 2017 tax data file can only be transferred from the computer where the tax data file is located in the Documents directory and the TurboTax folder when you are signed onto the 2018 online tax return.
See this TurboTax support FAQ for transferring 2017 tax data to the 2018 online return - https://ttlc.intuit.com/questions/1900883-how-do-i-transfer-last-year-s-return-into-turbotax-online
