The 2018 tax return that was filed during calendar year 2019. If you filed the 2018 as Married Filing Jointly the 2018 AGI is the same for the both of you.
The 2018 AGI is on the 2018 Federal tax return you filed Form 1040 Line 7
To access your current or prior year tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/
Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show. Click on the Year and Click on View adjusted gross income (AGI)
Or go to this IRS website for free federal tax return transcripts - https://www.irs.gov/individuals/get-transcript
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...
It is asking for the 2018 tax year AGI. So it should be what was filed in 2019. If you also used TurboTax when filing last year, it is on the first page of the saved file, in the section titled
"2018 Federal Tax Return Summary"
"Adjusted Gross Income"