If a previous year's AGI was rejected but is shown as blank on the return transcript, what are the next steps to fix or receive the correct number provided one exists?

If your tax return continues to be rejected with the 2018 AGI then enter a 0 (zero) for the AGI and e-file again.

If this is also rejected then you will have to print and mail the tax return.

See this TurboTax support FAQ for printing a tax return for mailing - https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online...

I first entered a zero as it would not allow me to input a blank box.