If the 2018 AGI is incorrect then the IRS will reject the tax return. You enter the correct AGI and e-file the tax return again.
No. If you used the wrong AGI it will be rejected. When you get the rejection notice you can correct it and re-file.
https://ttlc.intuit.com/community/agi/help/where-do-i-correct-my-agi-in-turbotax-online/00/26311