When did you file the 2017 return? Was it prior to November 2018? If so, use the AGI from the original return, not an amended one (if applicable.) If filed November 2018 or later, try an AGI of 0 (zero).
Yes, on the 2017 Form 1040EZ, the AGI is on Line 4.
If it was filed before November 2018, and if the actual 2017 AGI doesn't work, if you wish you could pull a 2017 tax transcript from the IRS to see what they are showing. That requires opening a free account at the IRS website.
http://www.irs.gov/Individuals/Get-Transcript
If you don't qualify to open the account, you can order one to come by postal mail in 5-10 calendar days.
If everything else has been exhausted, as a last resort before filing by mail, I would try the AGI of 0 anyway, since it can't hurt, and it has worked for some people.
"What if it gets rejected after I mail it? And what amount should be listed for the 2017 AGI then?"
When filing by mail, the 2017 AGI does not come into play. A mailed return cannot be "rejected" for such, since it is not efiled. Entering the 2017 AGI is only to verify an efiled return, not a mailed one.
Yes, if you have to mail the Federal return, then you also have to mail the state return. They have to be mailed separately since they go to different places. The Filing Instructions should print with the returns. It's a good idea to mail them by a method that has tracking such as certified mail, priority mail, etc.
