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.
To complete and file a 2019 tax return using TurboTax you can use either the online or desktop CD/Download editions.
TurboTax 2019 online editions - https://turbotax.intuit.com/personal-taxes/online/
TuboTax 2019 desktop CD/Download editions - https://turbotax.intuit.com/personal-taxes/cd-download/