You can complete the 2019 tax return at any time.
Note that when you are e-filing the 2020 tax return you will be asked to enter the 2019 AGI. Since the 2019 tax return will be processed late by the IRS, enter a 0 (zero) for the 2019 AGI
To complete and file a 2019 tax return using TurboTax you would need to purchase, download and install on a personal computer one of the 2019 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/
A 2019 tax return can only be printed and mailed, it cannot be e-filed.
