What should I do if my AGI was rejected? This is the first time I'm using TurboTax. Last year, I didn't filed late nor amended nor joined to anyone. Help!

Try entering 0 for the AGI.  The IRS was very behind last year and may not have entered it in the database in time to check.   

Or you mailed it, try putting in 0 (zero) for the AGI. And see, https://ttlc.intuit.com/community/rejections/help/what-if-i-entered-the-correct-agi-and-i-m-still-getting-an-e-file-reject/00/27031

How to correct the AGI
https://ttlc.intuit.com/community/agi/help/where-do-i-correct-my-agi-in-turbotax-online/00/26311

If you can’t get it to efile you will have to print and mail it.
How to mail a return from the Online version
https://ttlc.intuit.com/community/printing/help/how-do-i-print-and-mail-my-return-in-turbotax-online/00/26258

Be sure to attach copies of your W2s and any 1099s that have withholding on them.  You have to mail federal and state in separate envelopes because they go to different places.  Get a tracking number from the post office when you mail them for proof of filing.


For AGI Reject code 031
https://ttlc.intuit.com/community/rejections/help/e-file-reject-ind-031-04-or-ind-032-04-the-agi-or-self-select-pin-from-last-year-doesn-t-match-irs/00/27197