Why? It does not even matter now. The rejection code or message was in the email that you got when the return was rejected. If your 2020 return was rejected the only way to file it now is by mail. E-filing is closed permanently for 2020 returns. And most rejections are for incorrect AGI, which no longer matters since you only needed it to e-file.
Print, sign and mail your return. Expect about six months for the IRS to process it.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.
Federal and state returns must be in separate envelopes and they are mailed to different addresses. Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.