Should i file by mail when my tax return is rejected because of an AGI issue from last year? This is my first time filing and it says my AGI doesn't match from last year.

[mesquitebean]

"first time filing"
Was a return not filed in your name at all for 2017?   Or did someone else, such as a parent, file one last year in your name for you?

If there was no return in your name at all in 2017, either by you or a parent, then there is no 2017 AGI in the IRS database.  So you would use a 2017 AGI of 0 (zero.)

You can make the correction and resubmit the efile.