Be sure you are using the correct figure for the AGI from 2017. This FAQ tells you where to find it:
https://ttlc.intuit.com/questions/1900840-how-do-i-find-last-year-s-agi
Be sure you are using the AGI from the original 2017 return, not an amended one.
If you're married filing joinly, both spouses use the same 2017 AGI. Do not split or otherwise allocate it.
If you qualify to open an IRS online account, you could download a free 2017 tax return transcript from the IRS to see what they have in their database.
http://www.irs.gov/Individuals/Get-Transcript
If you filed real late in the year, such as November 2018 or later, then use an AGI of 0 (zero). Also, if you did not file a 2017 return at all, you would use an AGI of 0.
If still no luck, see this FAQ:
https://ttlc.intuit.com/questions/3415597-what-if-i-entered-the-correct-agi-and-i-m-still-getting-an...
However, before giving up and mailing the return, you can try the AGI of 0 (zero) as a last ditch attempt if you haven't already.
Otherwise, if all of the above has failed, then you will need to print, date, sign, and mail the return. TurboTax does not mail it for you. It's recommended to mail by a method that has tracking such as certified return receipt or priority mail, etc.
If you have both Federal and state returns, be sure to mail them in separate envelopes to the respective tax agencies.
FAQ: How to print and mail:
https://ttlc.intuit.com/questions/1944348-how-do-i-print-and-mail-my-return-in-turbotax-online