You can only enter a 2018 AGI if you filed a 2018 federal tax return. The 2018 AGI is located on the Federal tax return Form 1040 Line 7
A 2017 AGI cannot be used.
If you did not file a 2018 tax return then enter a 0 (zero) for the 2018 AGI.
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...
