2020 return was filed online but rejected by IRS because they had not processed 2019. Now I need to amend the 2020 and that option is not available.

If your 2020 return was rejected and you did not print and mail it instead, then it was never filed at all.   You do not "amend" a return that has never been filed.  Amending is a process you use for a return that was actually filed with the IRS and processed with mistakes or missing information.

Since you did not file, you can use the desktop software to make corrections, then print, sign and mail your 2020 return.  You do not need your 2019 AGI since you only need that for e-filing.

Online tax preparation and e-filing for 2020 returns is now permanently closed.  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 2020 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/questions/2559403-how-do-i-finish-a-prior-year-return-that-i-started-in-turbotax-online

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 9 p.m. Pacific time Monday-Friday.

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.

And....since you did not file 2020, the IRS does not have a 2020 AGI in the system.   You will not be able to e-file 2021 unless you use zero as your 2020 AGI.   However, midnight tonight is your deadline to e-file 2021, so if you have not done your 2021 return, you are going to have to send it by mail later, too.