you'll need to amend to take the deduction in 2019, if you do not those 2019 contributions made in 2020 will be treated as 2020 contributions which means the maximum additional amount you can contribute for 2020 will be reduced.
Go to the following link for instructions to amend a 2019 tax return that you filed with TurboTax Online.
An amended return has to be filed on a special form, Form 1040-X. It cannot be e-filed. You have to print it, sign it, and file it by mail. It normally takes the IRS up to 16 weeks to process an amended return. However, the IRS processing centers were closed for several months because of the Covid-19 pandemic, and they did not process any mailed tax returns, including amended returns. They are just now starting to process them again, but they have reduced staff and a huge backlog, so it will be a long time before your amended return is processed.
Don't file the amended return until your first return has been accepted and you have received the original refund.