If you have not filed for 2018 yet, then at this point you can no longer use an Online version of TurboTax.
You will have to use a TurboTax product for a previous year, which requires installation on a computer.
See this TurboTax support FAQ for completing a 2018 tax return originally started with the online editions - https://ttlc.intuit.com/community/filing-and-paying-taxes/help/what-if-i-missed-the-october-15-filin...
If you never started a 2018 tax return then if you choose to use TurboTax to create, complete and file the 2018 tax return you will need to purchase, download and install on a personal computer 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.