You need your 2018 AGI to efile 2019. The AGI is on 2018 1040 line 7.
https://ttlc.intuit.com/community/agi/help/how-do-i-find-last-year-s-agi/01/25947
Or see the screen shot by critter on this one,
https://ttlc.intuit.com/community/taxes/discussion/re-i-m-order-to-complete-my-taxes-it-is-asking-me-for-a-form-from-2018-that-i-don-t-have-and-i-don-t/01/995184
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
When did you file 2018? If it was late in the year try putting in 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.