If the IRS sends you interest for the delayed processing of your 2020 return, you are not going to receive that interest until sometime in 2022---so it does not have to be entered on a 2021 tax return. It will go on your 2022 tax return--next year.
Stop stressing over it and file your 2021 return if you are ready to file.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
