If no taxes are owed or you are receiving a tax refund there is no penalty for filing after the due date.
An extension request can no longer be submitted.
Since the IRS has rejected your e-filed 2020 tax return for the 2019 AGI, enter a 0 (zero) for the 2019 AGI and e-file the 2020 tax return again.
If the IRS also rejects the tax return for this 2019 AGI then you will have to print and mail the tax return.
See this TurboTax support FAQ for how to print a tax return for mailing - https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online...
