Is it possible to E-file if prior year return was sent in via mail? Current year tax return continues to be rejected because prior year AGI does not match E-file records

If you filed on time, your AGI should be in the e-file database, even if you filed by mail, but if you filed late, it may not be.

If you are sure you are using your correct federal adjusted gross income from 2017, and it is being rejected, try zero (in case your return was processed late and not entered into the e-file database).  

If that doesnt work, you will need to mail this year's return too.