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.
@Mehdimho You're posting in a 3-year-old thread that transferred into this new forum when it started a few weeks ago. All old threads were given an early June 2019 date when they were transferred unfortunately.
In regard to using a 2017 AGI to efile your 2018 return, be sure you are using the AGI from the original 2017 return, not an amended one.
If it's a joint return, use the same total AGI for both spouses. Do not divide it.
If you filed your 2017 return real late in the year, such as November 2018 or later, then try an AGI of 0 (zero.)
If you want, you can download a 2017 tax return transcript at the IRS website, to see what they show for your 2017 AGI.
NOTE: If nothing works, then before you give up and file by printing and mailing, try using an AGI of 0 (zero) sort of like a "Hail Mary" football pass. Even though I can't explain it, it worked for some people.