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...
Using my 2018 Turbo Tax file, I opened up the 1040 Forms and found the AGI on line 7.
Also using my 2018 Turbo Tax when you look at the Analysis and Advice tab under income, you will see a graph with vertical bars. Your AGI is listed there as well.