If you did not file a 2018 return then there is no 2018 AGI in the system for you. Try using zero. If zero will not work and let you e-file then your return has to be printed, signed and mailed instead.
And to finish your 2018 return ----
Online tax preparation and e-filing for 2018 returns is now permanently closed. You had until midnight on Tuesday, October 15 to finish and attempt to e-file; if your e-file was rejected you had until October 20 to make corrections and re-file. Now it is too late to use online software or to e-file. If you have any changes/corrections at all to make to your 2018 return, you now have to complete it using desktop software, which you must download to a full PC or Mac (not to a mobile device).Then you need to print, sign and mail it.
If you already paid for your online software you can ask customer support for a download of the desktop software.
They are available from 5 a.m. to 5 p.m. Pacific time Monday-Friday.
https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number
If you are getting a refund, there is no penalty for filing late. If you owe tax due, then file and pay the amount due as shown on the Form 1040, but expect a bill later from the IRS for the penalty and interest you will owe. Only the IRS will calculate this—TurboTax will not calculate it.