Entering your prior year AGI is a new IRS requirement for electronically filed returns, the best way to find your AGI is on last year’s tax return, follow the instructions here: https://ttlc.intuit.com/questions/1900840-how-do-i-find-last-year-s-agi
Did you amend your tax return last year? If so, make sure you are using your AGI from your original tax return, not the amended return. If the only form you have is the 1040X, this would be line 1, column A
Did you file last year’s return late or was it processed late (after mid-November 2016)? If so, you can enter zero as your AGI and then e-file again
If you want to double check the number on your tax return with the IRS to make sure that your printed copy is the same as what is on file, you can also request a transcript here: https://www.irs.gov/individuals/get-transcript If you can’t get your transcript online, you can also request one by mail.
If you still can’t resolve your e-file reject you can always print and mail your tax return(s) instead of electronic filing, you don’t need to provide your AGI to print and mail.