If your 2018 return rejected and was never filed then enter 0 for the AGI to efile 19 now.
Then for 2018 you will have to print and mail it now.
How to finish a prior year return you started online
https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-finish-a-prior-year-return-that-i-...
