The TurboTax online editions are Only for tax year 2019
To complete and file a 2018 tax return using TurboTax you would need to purchase, download and install on a personal computer, Win or Mac, 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.