No, you cannot add your W-2 for 2018 in the Online version.  The Online version is only for the current tax season (2019). You would need the download TurboTax software for 2018.  Prior year returns must be mailed.  You can purchase the 2018 download software from TurboTax, at the following website, or (check the IRS website for 2018 forms)

https://turbotax.intuit.com/personal-taxes/past-years-products/

If you already filed a 2018 return, to add the 2018 W-2, you would need to amend your 2018 return.  Amended returns must be mailed.

How do I amend a 2018 return in TurboTax?

https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/00/27579