After you file

It sounds like you didn't pull in your 2020 .tax file before trying to amend it. 

If you did your original 2020 tax return using the Desktop version, you can simply open your 2020 tax return and select “Amend a return that was filed and accepted.”

If you used TurboTax online to do your tax return, you need to download your .tax file, download the 2020 program and then search for your .tax file and open it before trying to amend it. 

Here is how:

1. Log into your TurboTax account.
2. On your Tax Home, scroll down to Your tax returns & documents, select 2020, then Amend using TurboTax Online.
3. Follow the steps exactly.
4. Download the .tax file for your 2020 return, which you can do from Your tax returns & documents. Do not try to open the .tax file yet.
5. Open the Desktop 2020 software. 
6. On the first screen, select Find a tax file. See image below.
7. On the screen OK, let's get a start on your 2020 amended return, select the reason you're amending and Continue
8. Answer the questions on the following screens. The Refund Monitor will reset to $0—don’t worry, this is normal for amended tax returns
9. When you reach Here's the info for your amended return, select Start next to the info you'd like to change, then select Done
10. Carefully answer the remaining questions to finish amending your return.

If you didn't use TurboTax at all in 2020, you will need to recreate the tax return that you filed in 2020 before trying to amend it.