If 2019 rejects because of AGI mismatch and you have not filed 2018 then then the 2018 AGI is 0 (zero).
TurboTax online and e-file for 2018 ended on Oct 15, 2019. 2018 (and earlier tax years) can now only be completed using the CD/download software installed on a Mac or Windows computer. The return must be printed, signed and mailed.
2016-2018 software can be purchased here:
It might if you owe additional 2018 tax.