When e-filing a 2018 tax return you have to enter the 2017 AGI (Adjusted Gross Income) from your filed 2017 tax return for your electronic signature.
To access your current or prior year online tax returns sign onto the TurboTax website with the User ID 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)
You can also get free transcripts of your federal return from the IRS which includes the AGI - http://www.irs.gov/Individuals/Get-Transcript
If you filed your 2017 tax return late in 2018 (November or later) or did not file a 2017 tax return, then enter a 0 (zero) for the 2017 AGI. If your tax return continues to be rejected by the IRS then your only other option is to print and mail the tax return.
See this TurboTax support FAQ for the procedure to print a tax return for mailing when using the online editions - https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online...
