There is no deadline for amending a tax return. The deadline is for getting a refund from an amended return. If your amended 2019 tax return shows a refund, you will not get the refund. But you can still file the amended return. If your amended return shows additional tax due, you have to pay the additional tax.
TurboTax no longer supports 2019. You will have to either prepare the amended return yourself on paper forms, as DoninGA suggested, or go to a local tax professional. Some other tax software companies might still support 2019, but using other software to prepare an amended return would be difficult because the original tax return isn't there. You would probably have to recreate the original return with the other software in order to amend it.
No matter what you do, your 2019 amended return will have to be filed by mail. The IRS no longer accepts e-filing for 2019.
