How do I file 2019 taxes with turbo tax?

You cannot do so online.  You must purchase and install the TT 2019 Desktop software, and file by mail.

TurboTax® 2019 Tax Software for Filing Past Years’ Taxes, Prior Year Tax Preparation (intuit.com)

Please see How do I prepare a 2019, 2018, or 2017 tax return? for instructions on how to prepare a 2019 tax return. 

You must purchase and use the desktop 2019 version of Turbo Tax(but not 2020, or 2018 versions, for example). Here is where you can purchase previous years' versions of Turbo Tax: https://turbotax.intuit.com/personal-taxes/[product key removed]cts/

You have to decide whether you need Turbo Tax Deluxe, or Premier, or Home & Business, or Business, but the above page explains which of these Turbo Tax products are most appropriate for a specific use case. Just make sure that for the product that you select, the version/year is 2019. Turbo Tax can then guide you to file online or via mail.