Probably because they haven't processed last years tax returns yet. If you enter "0" in AGI instead of the correct number, it should be accepted. If they haven't processed your 2019 returns, to them, your AGI is "0." As stupid as it sounds, it worked for me.
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I followed Turbo Tax path to fix it and was rejected twice. That's when I figured the Feds hadn't processed last year's returns yet, so I entered "0" for AGI and it was accepted. If you find you are following Turbo Tax advice and entering the correct AGI from last year and you are getting rejected... stop following the Turbo Tax advice and put in "0" for AGI and try it again. I found, personally, following Turbo Tax advice on this specific issue only took extra time, exhausted my tolerance and took me to the same dead end since the advice is standard and not specific to the issue at hand. I love Turbo Tax, but was really disappointed on this known issue that should have a known response.