Were you able to open the original return in the 2021 program? Make sure you start with the original return.
Then you first need to check the filing status inside the program so it knows it was filed and accepted. Open your return and go up to the top black menu bar first item
File > Electronic Filing > Check Electronic Filing Status
Then save your return and close it. Close the program and reopen it.
Then Go to:
Federal Taxes -or- Personal (for H&B)
Other Tax Situations
last section, Other Tax Forms
Amend A Return - Click the Start or Update button
Then enter your changes.