Your 2016 tax return shows your 2016 Adjusted Gross Income (AGI) If you filed a joint return then the AGI is the same for each of you.
Form 1040EZ line 4
Form 1040A line 21
Form 1040 line 37
If you are using the same TT account that you used last year, you should also be able to find this number in your Tax Timeline for 2016
If entering the amount from your 2016 tax return does not work, then try entering “0” instead. If that does not work and you still cannot e-file, then print, sign, and mail your tax return.
You can get a free transcript of your past return from the IRS:
Note: If you filed late or amended your return, your correct 2016 AGI may not be in the system. The IRS database may have errors for 2016 AGI's. A number of people have experienced rejections from the IRS even when entering correct AGI's from their 2016 returns. When it happens, you just have to mail the return.
Note: If you mail your federal return, you will also need to mail your state return. You will not be able to e-file your state return.