You can only file a 2018 W-2 with a 2018 tax return or a 2018 amended return if you already filed 2018. 2018 can only be done using the 2018 desktop software and mailed.
You can purchase 2018 software here.
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.