The current TurboTax Online system is only for 2019. You cannot do - nor should you add - your 2018 return on the current Online product.
If you entered 2018 data and immediately thereafter entered 2019 data, then you overwrote your 2018 data with the 2019 data, which could be OK if no 2018 data was left. But you must not add 2018 W-2s and 2019 W-2s to the same return. If you filed your 2019 return, you may have to amend it to remove any traces of 2018 data.
Please see this TurboTax FAQ for information on how to file your 2018 return.