It's not clear that you have to make any adjustments or changes to your tax return. If the IRS is correct, maybe all you have to do is pay the amount that you owe. Read the instructions in the notice for what to do if you agree with it.
If you do have to, or want to, revise the tax return for some reason, you can no longer import 2019 brokerage information. You would have to enter the missing information manually from the printed or PDF statements. And you cannot do this in TurboTax Online. You would have to use the CD/Download TurboTax software for 2019.
You should also check your 2020 tax return to see if the same error occurred. If so, you can file an amended return for 2020 instead of waiting until you get an IRS notice for 2020.
