The 2018 AGI is on the 2018 Federal tax return you filed Form 1040 Line 7
https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/01/25947
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...
Even if you used turbotax last year and filed you may not have created an account. I just did this, go to the official IRS website and create an account and right after it's created the home page of your account on the IRS website gives you 2018's AGI right there. I was freaking out about this too but it took 5 minutes to create an IRS account and get it electronically.