You have to enter the 2017 AGI as shown on the 2017 tax return -
Form 1040 – Line 37
Form 1040A – Line 21
Form 1040EZ – Line 4
If you filed a 2017 late in 2018 or you did not file a 2017 tax return, then enter a 0 (zero) for the 2017 AGI and e-file again. If the IRS rejects the tax return again then you will need 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...
If you have taxes owed on the 2018 return then include a copy of the first rejection email you received when mailing the federal tax return so that you will not be penalized for filing after the due date if you filed an extension request.
