Discovered a past year's return was rejected. How do I see why and fix it?

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When the return was rejected, you received an email telling you why it was rejected.  There was either a rejection code or a rejection message.   Many rejections are for AGI, which is only needed in order to e-file.   AGI no longer matters if you are trying to file a past year return.  A past year return can only be filed by mail; it cannot be e-filed.   You will need to print, sign and date it in ink, and mail it.   You have not mentioned what tax year the return is for.   If it is for a year earlier than 2022, the refund has been forfeited and will not be paid.   If you owe tax due, it is owed with late filing penalties and interest, for which the IRS will bill you later.

Now it is too late to use online software or to e-file. If you have any changes/corrections at all to make to your past year  return, you now have to complete it using desktop software, which you must download to a full PC or Mac  (not to a mobile device).Then you need to print, sign and mail it.

https://ttlc.intuit.com/turbotax-support/en-us/help-article/printers-printing/finish-prior-year-retu...

If you already paid for your online software you can ask customer support for a download of the desktop software.

They are available from 5 a.m. to 5p.m. Pacific time Monday - Friday Pacific time.

https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number

If you are getting a refund, there is no penalty for filing late.  If you owe tax due, then file and pay the amount due as shown on the Form 1040, but expect a bill later from the IRS for the penalty and interest you will owe.  Only the IRS will calculate this—TurboTax will not calculate 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.