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...
did you file 2018 after 10/15/2019 or amend your 18 return?. if amended, use AGI as per the original return. if you filed after 10/15 then try zero. sometimes nothing works and you will have to file by mail.