Since a 2018 tax return can only be printed and mailed to the IRS there is no need for a 2017 AGI.
You are using the TurboTax 2018 desktop software and not the TurboTax online editions which are only for tax year 2019, correct?
To complete a 2018 tax return using TurboTax you will need to purchase, download and install on a personal computer one of the 2018 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
A 2018 tax return can only be printed and mailed, it cannot be e-filed.
