You only need to attach the 1099 if it shows withholding on it. They usually don't unless it's a 1099R.
If you made any payments when you filed the extensions or as estimates payments do the amounts show up? For 2020 Federal Extension payments should be on schedule 3 line 9. And go to 1040 line 31.
For 2020 Federal Estimated payments should be on 1040 line 26.
What did the IRS change on your 2019 return? They should have mailed a letter. Did you forget to enter some payments you made?
