Are you referring to your 2019 or 2020 tax return?
When e-filing your tax return, you should not need to mail anything in unless the program instructed you to do so. Please see Do I need to mail anything after I've e-filed? for more information. If you are unsure if the IRS needs any additional information on a past return, you will need to contact them directly. Please see How do I contact the Internal Revenue Service (IRS)? for instructions.