If you never filed a 2018 tax return then for the 2018 AGI enter a 0 (zero) 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...
