If it was rejected for AGI—-be aware that many 2019 returns took so long for the IRS to process that your 2019 AGI might not be in the IRS system. Try using zero instead.
If filing with zero is rejected then file again but select the option that you did not file last year. The IRS does not see that question and it will allow you to e-file with no AGI question.
Or—-if you filed as a “non-filer” just to get the stimulus check on a 2019 return, try using $1 as the AGI.
If that does not work and you still cannot e-file, then print, sign, and mail your tax return.